Cycling route along the Weser river with a length of about 500 kilometers starts in Hann. Münden and continues to Cuxhaven at the North. And then the cycle path in the framework of the European route R1 goes through the territory of Poland to St. Petersburg.
For those who's just not familiar with bicycle paths in Germany, I will describe what it is. This is a completely paved road, about 2 meters wide. Along the route you can find benches and gazebos for relaxing. There are cafes, restaurants and hotels every 3-5 kilometers. There are pointers with distances near every intersection. Although it's not possible to get lost on this bike path as it runs along the river.
If you have one day, just select the area between the towns with the train stations to go home in the evening. I did so and traveled about 67 km from Hoexter to Hamelin. I should say that it's not easy for the untrained. So, if you want to feel what is the bike path as a tourist, do not select a route with a length of more than 25-30 kilometers.
The route is generally flat, ideal for cycling with children, and very popular among cyclists.
What you will see on the road (from the places that we have already visited):
Hann. Münden: Here the Fulda and Weser rivers merge, the start of the route, a great old city
Corvey Castle: Ancient abbey with the oldest Westerwerk in the world
Holzminden: A lot of new buildings in the old city. I only had a lunch here, but in general you can walk in the city.
Bevern Castle: Aside from the bicycle path, in Holzminden you should turn to Bevern
Everstein Castle in Polle: A place from the Cinderella tale, great view of the Weser from the top of the tower
Bodenwerder: Hometown of Baron Munchausen
Hämelschenburg сastle: From Emmerthal you'll need to make a 5 km detour to the castle
Hamelin: City of Pied Piper
Porta Westfalica: A monument to Kaiser Wilhelm stands on the hill, offering great views.
Minden: A lot of new buildings in the old city, but there is a water bridge!
Bremen: The central square is on the UNESCO list.
There are a lot of websites dedicated to this bike path and all of them call themselves official, pick any :)
http://www.weserradweg-info.de/
http://weser-radweg.de/de/
http://www.weser-radweg.com/de
Getting there: Choose any city with a railway station on the route.