If you should go to Bodensee / Switzerland, don't miss the Rhine falls at Schaffhausen (they may be quite spectacular for the kids).

In Heidelberg, the zoo (in the Neuenheimer Feld) is quite nice. You can take a boat tour at the Neckar river (although the Rhine valley between Mainz and Koblenz is more spectacular).

There is a castle close to Heidelberg which has falcon "shows". Unfortunately I don't remember the name.

If you're there in summertime, the public pool in Leimen (just north of Heidelberg) is a nice spot to take a bath.

Just some ideas for kids.