I would certainly second the recommendations about the Porsche Museum in Stuttgart (yes, heresy I know, but they have some cool cars, and the building is pretty nice too)
Van Gogh in Amsterdam for sure, and (supporting our past president the esteemed Mr. Van Es - Rijksmuseum which is a short walk away and the world class car museum, Louwman museum in The Hague is well worth a stop.)
Not that much further a drive from Fellbach (the Classic Center) is a visit to Afalterbach - AMG
We just love to spend other folks money.
You pretty much can't make any really wrong choices.