Try to make it down to Freiburg for a taste of medieval German city and the Black Forest. Its beautiful and it really does feel like stepping into another time.
(Of course you might could just keep heading south and spend the rest of your vacation in Florence gorging yourself on delicious food and getting smashed on Brunello)
I've been to Amsterdam a few times but... errr... I don't really remember it. The Van Gogh museum is one of my favorite art museums, but I am not much of an art lover.