If you're a car buff, the Mercedes Benz museum in Stuttgart is worth a visit if you're passing through. Heidelberg is also pretty accessible too and has a really great gothic feel, there's a really cool castle there and a great nightlife. You can take the S-bahn between any of the cities from Dusseldorf to Frankfurt, I find myself lumping them together since they're really part of one huge (albeit segmented) metropolitan area.