Dan
Go see the Porsche museum next to the factory in Porschestrasse 42 (duh), Zuffenhausen, the MB museum in Mercedesstrasse 137 (double-duh), Unterturkheim, and if you like the outdoors, go take a boat trip on the river Neckar after a stroll through the Rosensteinpark, both in the Bad Canstatt area. Take a walk through the main shopping district in Königstrasse and the Calwer passage in the centre of the city, and drink a proper weizenbier or three (Erdinger) at one of the many streetcafes in the Schlossgarten area also in the city centre. You can also go up to the viewing deck of the Fernsehturn (tv tower) up on the hill in the Degerloch area. On a clear day, you will have quite a view.
If you are into old castles, go see the Schloss Solitude towards Leonberg.
Try to eat the local specialities, such as Maultaschen, or one of the many Spätzle dishes, a handmade type of noodle. I recommend Spätzle with Leberkäse. There are many microbreweries in the city centre, try a few! A well-known one is the Calwer-Eck-Bräu near the Calwer passage shopping area. You may need a reservation, though.
Enjoy your trip!!
(I used to live there, can you tell?)
LeRoux