Plansee, Austria, just across the German border. (Seen from Geierköpfe summit)
(Well not actually the place where I live, but close enough to go there for half a day.)
Beautiful scenery all around, and makes for a refreshing swim in summer after hiking/climbing in the surrounding mountains. It's also close to the touristy castles (Neuschwanstein, Linderhof), so it's easy enough to fit in while visiting the south of Germany.