If you want some flat and relaxing, I would recommend the ride from Romanshorn in Switzerland to Konstanz. Beautiful scenery along lake Konstanz mostly on a separate bike path. There is a great little stand that sells fresh apple juice and other good stuff somewhere, along there I would recommend for a break. And Konstanz is an enjoyable place to stop for lunch. I think it is only 25 km, but you can always cycle back.

Probably not your traditional mountain scenery, but a nice day out nonetheless. May be able to extend if further around the Lake. Lindau and Meersburg (need to take a boat there are well worth the visit also)