in frankfurt museum fur moderne kunstthe aka "the slice of cake." nice building and one of my favorite modern art museums.

in stuttgart, you can see the classic Weissenhofseidlung
housing ensemble, featuring Mies, Corb, Scharoun, Oud and others. only mildly interesting for those who are not history / modernism buffs.

stuttgart: Porsche factory tour! you can try to reach the museum and they can tell you how to join a tour, or contact your local porsche dealer (no purchase required)

Stirling's Neue Staatsgalerie is great. Good winking counterpoint to all that earnest Modernism at the Seidlung.