I'm guessing you're based in England. Are you?

Any river route in Germany will be easy and romantic. The Moselle is a top one, as is the Danube. The Main (where I am) is also great. There are literally too many to list here, but most of them are flat and scenic.

Are you set on Germany or German speaking areas? In my not-so-unbiased opinon, Germany's bike routes are the best.