Bremen is on the Weser River, which has a gorgeous bike path going from the North Sea all the way down to the Fulda River, which will take you towards Frankfurt.
I did that with my wife in August. Beautiful riding! Lots of great sights to see as that route is also known as the "Fairy Tale Route". You will pass through the town the Pied Piper rid of rats. Lots of other famous Brothers Grimm fairy tale town along that route as well.