We just got back from a trip to Germany, we did the MB Museum, Porsche Museum and BMW museums all are brand new and amazing and are worth visiting if you have time. Definitely go to the Benz museum as it is probably the most spectacular overall.
We stayed at the hotel Azenberg in Stuttgart, not walkable but an excellent little boutique hotel with a great pool and spa included, and the most enormous breakfast that was also included with the room, but the hotel is about a 15-20 minute drive from Zuffenhausen.
http://www.tripadvisor.com/Hotel_Review-g187291-d232577-Reviews-Hotel_Azenberg-Stuttgart_Baden_Wurttemberg.html
Have fun!