With its half-ruined castled dominating the heights above the university-town on the banks of the Neckar river, Heidelberg is a pedestrian's dream. "Picturesque" may have become a cliché, but this German city exemplifies the meaning. I'd been here as a small child--I distinctly remember walking up the stepped road to get to the castle--my 'first castle'--and the stairs seemed endless...Leave your car behind--walk or bike around the historic center--it's a Baroque jewel. And when the sun comes out, soak it up.
(You can get to Heidelberg directly from the Frankfurt airport, via either bus or train--from 45 to 80 minutes...)