Heidelberg's OK, but I'd certainly second Bamberg and Regensburg.

Mary Fran: there's only one way to see Rothenburg - stay overnight. My first time was a day trip and my impression was much like yours. Next time I stayed over, and it was magic. The transition was half the impact - sit in a corner of the main square with a beer (there's a tree-shaded biergarten there) and watch as the crowds and tour buses melt away and the locals start coming out to play. What seemed an over-crowded picture-book fantasy town transmogrifies into a living, breathing real town right before your eyes.

Postal: A nice daytrip would be northeast of Bamberg about 30km: Vierzehnheiligen (Baroque pilgrimage church) and Kloster Banz (Baroque monastery and church).