Germany, Ich Komme Zurück!

I am scheduled to fly back to Germany on the 10th of March for about 6 weeks. I will be staying in Frankfurt, like the last time. Weekdays will be hectic at work, but I ought to have the weekends to myself. Any suggestions on which cities or towns within the European mainland (within the Schengen Agreement member countries, preferably) are worth a visit?

I visited Paris (hated it, except for Jim Morrison’s grave) and Rome (loved everything about it) the last time I was in Europe, so those two are out. I’m thinking Amsterdam and Prague this time. What do you’ll say?