Lake Constance

You have to go to Meersburg, especially the castle, possibly the oldest in Germany. It's a real castle, actually lived in and actually used (as opposed to Neuschwanstein). Insel Mainau (island Mainau) is great in the summer, beautiful gardens, although the palace is still lived in and not open for tours. Lindau is another cute town with lots to see. Make good use of the ferry system, it'll save you time.

I'd figure out early just how completely you want to go around the lake. There are two arms of the lake approx. at Überlingen and Radolfzell. If you don't want to go around the southern arm, through Radolfzell, there is a bridge at Konstanz, but you'll have to use a ferry at Mainau to avoid the northern arm.