Do they have designated rural cycle routes that take you to sites and places of interest around Hamburg? A coastal route would be lovely.
How about this one?
It's a bit long though, and you'll end up quite some distance away some two or three weeks later!
Well, that's a little more than I was intending to cycle. I wanted to spend the night in my own hotel bed in Hamburg ideally, but great recommendation none-the-less. I bet that would be a fantastic journey.
I thought it might be truth-be-told. You could alway just cycle part of it and then turn round and come back.
If you check out the German Cycle Association website they have meets and lots of information
They also sell maps for cyclists with marked routes wherever you want in Germany.