I'm going to be cashing in some miles and flying to/from FRA this Dec (2-14) and will be staying for 12days/11 nights. A few yrs ago I did a whirlwind tour (Rothenburg, Fussen, Zurich, Zermatt, Grindelwald, Bern, Baden-Baden, Heidelburg). This time I'm looking for something a little less hectic.

I was thinking of using Munich and Bern (Switzerland) as bases for 3 nights each. Then maybe 2-3 days in Salzburg and/or Innsbruck. I'd like to explore some part of SW Germany for a night or two as we return back to FRA to break up the lengthy rail trip from Bern. Any nice towns easy to get to via rail that would be worth visiting in December?

I'm just trying to figure out some options and logistics of getting around most efficiently w/o a car while enjoy the sights (castles, churches, markets, museums).
Thanks.