Why don't you go to an FKK? Big ones in Frankfurt, in Karlsruhe you've got Flamingo Island. In Augsburg Colosseum. Even if you do not have a car, Mannheim is one of the best rail hubs in Germany, and will get you there pretty quickly. Plenty of info on the board, if you search.
On average you'll pay EUR 50-75 for entry, but after that it's EUR 50 for each half hour session, and you've got more eye-candy than most of can handle.