I loved Heidelburg - great university town, and small enough to walk pretty much anywhere. I was there for a conference that had a dinner/reception inside the castle - it was one of the best meals I ever had. Awesome place!
Yup.
One place I forgot to mention, but it's worth the pilgrimage (it's kinda out in the sticks, as Germany goes) is Kloster Kreuzberg, a beautiful monastery that pays for its upkeep through a small restaurant and brewing the greatest beer on the face of the planet. Great schnitzel to be had, as well.
I know, I know, "Steve, it's not really the best beer on the planet!" But it is. It's absolutely amazing. The beer is hoppy, strong, fresh, and flavorful. It'd divine. If you go to Germany, go to Kreuzberg.