Back to the topic at hand...
+1 on the Mercedes museum. Fantastic.
+1 on Ludwigsburg. The palace there was worth a stop.
You're an hour or so away from the museums at Sinsheim and Speyer. If you haven't been to these before, they are excellent destinations.
Others have come up with travel suggestions, but you're not far away from the black forest.
For other car-related destinations in southern Germany, this book is tough to beat for more ideas:
Via Corsa Car Lover's Guide to Southern Germany: Ronald Adams: 9780982571019: Amazon.com: Books