Because Germany is such a bike-friendly country there are many possible cycling routes for the visitor. The German Tourist board maintains information for more than 80 such paths. Of these we have identified a few long flat routes which you may wish to investigate.
If you want to see any smaller ones have a look at the Bike brochure at German Tourist Information. The websites above are the official ones although some commercial organisations (providing bike hire and luggage transport) can be found using Google. It is worth just checking the German version of these websites as often details are missed off the English translation.