Hallo Reza,
to make up your mind where to cycle in Germany have a look on
http://www.esterbauer.com/index.html> bikeline RADTOURENBÜCHER
> Deutschland
click the map
and you will find severall routes in Germany and information about each.
Have a look and contact me in case of questions i'll try to help you.
I'm thinking of cycling in summer, too. I live near the dutch boarder in Mönchengladbach.
Good luck
Achim