In southwestern Germany, Lindau is worth a visit and at least a full day. I recently stayed at the
Bayerischer Hof and it is right on the Seepromenade.
If we move away from the southwest, the OP might enjoy southern Bavaria and the Garmisch-Partenkirchen area. I was just there a week or so ago and stayed at a wonderful 5* resort/spa,
Schloß Elmau, which I would highly recommend.