For the boys: Technikmuseum Sinsheim, new Mercedes-Benz Museum in Stuttgart, maybe Technorama in Winterthur/CH and Verkehrshaus in Luzern/CH.

For your rental car: If actually going to Switzerland, refuel only at swiss gas stations (Gas about 0,25€ cheaper per liter than in Germany)

For the view: Bodensee (Lake Constance), Schwarzwald (Black Forrest) and Lake Zurich are a must.

I'd visit Strasbourg, Colmar, Stuttgart, Baden-Baden, Freiburg, Konstanz (cross the border here), then Zurich, Luzern and Sankt Gallen. You could then take the ferry from Romanshorn/CH to Friedrichshafen/D.

PS: Keep in mind that you need to pay a higway fee both in Austria and in Switzerland, if you're catched without paying they'll charge you at least 125€ (in Austria).

I wouldn't spend to much time in the Heidelberg area, IMHO it doesn't offer that much. If you're going during the summer you can jump in the Bodensee, very clean water and a nice refreshment.