Danube - second largest river in Europe (first is Volga), with a length of approx. 2850 km. Theoretically Danube bicycle path (Donauradweg) begins in the German city of Donaueschingen and flows to the Black Sea. In practice, the marked route ends more or less in Budapest (however earlier are also some unfinished route sections). Next, you have to consider the ride normal roads (I mean full of traffic) and poor trail markings. We cycled the section of Passau - Budapest, about 600 km long route along the Danube river. This is supposedly the most popular fragment. Generally, you can drive both the northern and the southern riverbank.
Along the route there are numerous facilities for cyclists: campsites, rest areas, bicycle ferry crossings, taps with drinking water etc.
Germany, Austria
We start cycling from Nove Udoli in the Czech Republic. From here, there are a few kilometers to the German town Haidmühle. 
It is an area of the Bavarian Forest (mountain range has the same name: Bavarian Forest). We spend the night there and the next day we cycled to Passau. At the beginning the route is nice - hilly and not very busy. Near the town Waldkirchen, the number of cars began to increase and it was necessary to think about another way. Finally, instead of about 50 km, was well over 80 km and we reached Passau in the evening. 
In Passau, we stayed in cyclist hotel Rotel Inn in Germany. I do not recommend for people suffering from claustrophobia - the whole space of a tiny room is occupied by a bed. From Passau to Austria it is very close, the Danube is a border. We started cycling on the German side - the northern shore. After about 30 km, we were in Austria (Engelhartszell an der Donau).
We cross the river by ferry to the south bank and stayed on the campsite in Schlogen. Next day we went to Linz. Generally, the first stage of the Passau - Linz route - about 100 km is very varied and interesting. You're going through a deep river valley surrounded by high hills, passing by many bends of the river. The closer to Linz, the route becomes more monotonous. 
We slept near Linz on the roofed train stop - sometimes such situation may happen :). The next day we passed Linz quite fast. The cycling route passes along the Danube and bypasses the city center. Then continue along the north bank towards Grein.
Along the way we passed through Mauthausen, unfortunately known for the nazi concentration camps. We spend night at a campsite in Grein. It is worth visiting the Greinburg castle: a maritime museum, the oldest inhabited castle in Austria. By the way, a practical note: on the campsites, there are coin-operated washing machines, but you have to throw in exact required amount of money (usually 4-6 Eur). Machines do not give the rest, so if we throw in too little or too much,
it will be lost. The next day we're going to Ybbs. I don't recommend the north bank route (we were driving like that), because it is
marked along with a quite busy road (with a separate traffic lane for cyclists). Interestingly, along the way, we saw the signage:
Romantische Straße - it was really funny in such environment.
We decided to go to Ybbs and find the train station to take the train out of the busy road.
Along the way (in Ybbs), we found the tourist information point, and asked. Very kind boyfriend advised us not to use the train, printed the addresses of campsites on the way up to Bratislava (marking which are the best) and explained that it is definitely better to drive the south bank and that we will be satisfied. It actually was. From Ybbs we went to Melk, a town known as the "Wachau Valley Gate".
In Melk is the largest Benedictine abbey in Austria. Then we stayed for a night in the campsite in Schönbühel.
The next day we continued our way along the Wachau valley, the southern bank of the Danube towards Mautern
(on the northern shore there is a larger town - Krems) and then Tulln.
On the way, on the left bank of the Danube, we pass the picturesque town - Dürnstein, known for its narrow streets. You can get there by water taxi. We enjoyed the views from other shore. Above the town there are ruins of a castle, in which in the years 1192 - 1194 the King of England, Richard I, also known as Richard the Lionheart, was imprisoned.
He was captured during the return of the third crusade, for desecrating the Babenberg flag.
Next to the water taxi stop, there is a mini bar where you can order fast-food dishes and local alcoholic beverages,
especially the region's famous apricot alcohol.
We wanted to reach Tulln that day, but the afternoon heavy rain stopped our plans. We stayed at the campsite in Zwentendorf and it would be quite ok, if not the meeting of trans, techno fans or something like that (we don't like such kind of music), who arrived by cars and slept at this campsite too and were very noisy. Every tent had its own music equipment and they turned on music all at once. We were a little worried but fortunatelly after 1am it got quiet. The next day, without special sightseeing, we went through Tulln, Klosterneuburg, Vienna.
Through Vienna there are many bicycle routes, so you can cross the center. We chose eurovelo 6, which runs through Vienna on the banks of the Danube. Along the way, we had interesting contrasts, first Muslims, then the Nudist area on the Danube. We decided to go to Orth (over 30 km from Vienna) and look for accommodation there. The bicycle route here runs at a certain distance from the Danube. It is also worth mentioning that there are practically nothing for these 30 km, no supermarkets, only fields and fields, and a few collective-style buildings. There are several agro hostels in Orth, but there were some problems with finding accommodation at a reasonable price. Finally, the owner of one of them (in which there were no places) helped us - she called her friends and found for us an accomodatiom for 25 Euros per person (with breakfast). So after about a week of sleeping in the tent, we spent night in a comfortable bed. The next from Orth we went further on route eurovelo 6. 
A few kilometers from Orth, we entered the national park "Donau-Auen National Park", along the way we also pass the castle in Eckartsau.Then the route leads to the bridge over the Danube, which we passed to the south bank and went towards the Austrian-Slovak border. The last Austrian town which we passed was Hainburg. Just a few kilometers further, you can see the castle in Bratislava.
it will be lost. The next day we're going to Ybbs. I don't recommend the north bank route (we were driving like that), because it is
marked along with a quite busy road (with a separate traffic lane for cyclists). Interestingly, along the way, we saw the signage:
Romantische Straße - it was really funny in such environment.
Along the way (in Ybbs), we found the tourist information point, and asked. Very kind boyfriend advised us not to use the train, printed the addresses of campsites on the way up to Bratislava (marking which are the best) and explained that it is definitely better to drive the south bank and that we will be satisfied. It actually was. From Ybbs we went to Melk, a town known as the "Wachau Valley Gate".
In Melk is the largest Benedictine abbey in Austria. Then we stayed for a night in the campsite in Schönbühel.
The next day we continued our way along the Wachau valley, the southern bank of the Danube towards Mautern
(on the northern shore there is a larger town - Krems) and then Tulln.
He was captured during the return of the third crusade, for desecrating the Babenberg flag.
Next to the water taxi stop, there is a mini bar where you can order fast-food dishes and local alcoholic beverages,
especially the region's famous apricot alcohol.
Slovakia, Hungary
On the eighth day around noon we entered Slovakia. We continued to follow the eurovelo 6 route. We crossed the border next to MYSUM B-S 4 (http://mpo.sk/museum/). Bratislava has about 4 times less inhabitants than Vienna, which is visible during a city trip. Our route eurovelo quickly leads us to the urban, crowded boulevard.
You have to watch out for rollers, skaters, people with stroller and others like these. An interesting attraction and a place of rest and meetings are 8 (if I remember correctly) buffets located every few kilometers along the route.
For us, after a week of eating only supermarket food in Austria - something like titbit. Immediately we sat down and ordered "utopience" (thick sausages served sour with sauerkraut and bread). The issue is, that the prices of gastronomic services in Austria and Germany are too high for polish
people with average earnings (prices more than four times higher than in Poland).
On the same day, we decided to go to Gabcikovo. As it turned out later, that day we traveled about 150 km (Orth - Bratislava - Gabcikovo). Behind the last buffet, the boulevard ends and You have to look for eurovelo signs. The trail after a few kilometers leads us to the riverside road (asphalt road). This way we got to the bridge - You have to turn left to the bridge to go to Gabcikovo (2 - 3 km).
Interestingly, this part of trail is around twenty kilometers long, You just cycle and cycle like neverending riverside road, the Danube is wide enough that You almost can't see the other bank. It turned out that there is a dam called in Slovak "Vodné dielo Gabčíkovo". Finally, we arrived around 10:00 p.m. and found accommodation in a guesthouse in Gabcikovo (there are several accommodation options in the town, you only have to ask).
The next day we are going to the direction of Sturovo ( in Hungarian Parkany).
For about 30 km from the start, the good quality trail continues along the Danube, and then road surface turns into a volatile gravel - in such way You travel very slowly,
so we decided to leave trail and turn on the asphalt road towards city Komarano.
We asked in the city - it turned out that a quite long part of eurovelo 6 trail was made towards Sturovo (around 30 km). On that day we did not reach Sturovo and stayed a few kilometers before in the village called Muzla (in Stara Krcma - hostinec). The border town Štúrovo is known primarily for two battles fought by Jan III Sobieski with Turks in 1683.
There is a monument of the Polish king and a street named after him. Štúrovo with Hungarian town: Esztergom is connected with Maria Valeria bridge. Esztergom is one of the oldest cities in Hungary, mainly known from the Basilica of Saint Wojciech - the largest Catholic cathedral in Hungary. 
A great stuff, is the possibility of a cruise on the Danube. We decided to take a two-hour course to the Visegrad (it seems that you can get to Budapest, but you have to check it ). What's important, ships take bicycles and are not too expensive.
Details can be found at: http://www.mahartpassnave.hu/en/. So, by the river we got to Visegrád, a town known for meetings of The Visegrad Group, that is, the presidents of Hungary, Czech Republic, Slovakia and Poland (to commemorate the XIV century Visegrad congresses , meetings of the kings of Hungary, the Czech Republic and Poland) . We planned a further ride, but unfortunately came a huge storm. At the first big drops of rain we managed to hide in the Visegrad hotel. 
We had a nice surprise there, the boss of the hotel came to us and started talk to us in Polish. Finally He offered us an accommodation at affordable price. So we stayed overnight in the 4-star hotel in Visegrad.
Hotel website: https://www.hotelvisegrad.hu/en/. The next day we wanted to reach Budapest.
From the Visegrad, You have to follow 6 eurovelo route, sometimes along the road, but
there are also parts specially designed for cyclists.
For us, after a week of eating only supermarket food in Austria - something like titbit. Immediately we sat down and ordered "utopience" (thick sausages served sour with sauerkraut and bread). The issue is, that the prices of gastronomic services in Austria and Germany are too high for polish
people with average earnings (prices more than four times higher than in Poland).
On the same day, we decided to go to Gabcikovo. As it turned out later, that day we traveled about 150 km (Orth - Bratislava - Gabcikovo). Behind the last buffet, the boulevard ends and You have to look for eurovelo signs. The trail after a few kilometers leads us to the riverside road (asphalt road). This way we got to the bridge - You have to turn left to the bridge to go to Gabcikovo (2 - 3 km).
Interestingly, this part of trail is around twenty kilometers long, You just cycle and cycle like neverending riverside road, the Danube is wide enough that You almost can't see the other bank. It turned out that there is a dam called in Slovak "Vodné dielo Gabčíkovo". Finally, we arrived around 10:00 p.m. and found accommodation in a guesthouse in Gabcikovo (there are several accommodation options in the town, you only have to ask).
For about 30 km from the start, the good quality trail continues along the Danube, and then road surface turns into a volatile gravel - in such way You travel very slowly,
so we decided to leave trail and turn on the asphalt road towards city Komarano.
We asked in the city - it turned out that a quite long part of eurovelo 6 trail was made towards Sturovo (around 30 km). On that day we did not reach Sturovo and stayed a few kilometers before in the village called Muzla (in Stara Krcma - hostinec). The border town Štúrovo is known primarily for two battles fought by Jan III Sobieski with Turks in 1683.
A great stuff, is the possibility of a cruise on the Danube. We decided to take a two-hour course to the Visegrad (it seems that you can get to Budapest, but you have to check it ). What's important, ships take bicycles and are not too expensive.
We had a nice surprise there, the boss of the hotel came to us and started talk to us in Polish. Finally He offered us an accommodation at affordable price. So we stayed overnight in the 4-star hotel in Visegrad.
Hotel website: https://www.hotelvisegrad.hu/en/. The next day we wanted to reach Budapest.
From the Visegrad, You have to follow 6 eurovelo route, sometimes along the road, but
there are also parts specially designed for cyclists.
