Colonge (Koln)- worth a look, the beer is goooooood, the church/chapel thingy is incredible and the train from Frankfurt rocks along at 300km/h or so.
Also Toyota Team Europe http://www.toyota-motorsport.com/en/ is based there, so you could pose for a couple of pics or see if you can look inside to check out some of the old race cars. I managed a tour of the facilities back in the F1 days, but that was a one off I think :P
Another option is the Micheal Schumaker Go Kart Centre http://www.ms-kartcenter.de/com/index2.php. Worth a blast. Still have the Micheal Schumaker Champers sitting on the shelf that I won, and it was a bastard getting the trophy back home......... :D