If you've got a bit of money then The American Hotel sounds appropriate for a 25th Anniversary. I've stayed a couple of times and it's a great Art Deco hotel close to the Rijksmuseum. I think it was Hitler's HQ in Holland during the war.

Obviously you'll be doing the Van Gogh museum but the Concert Hall is also world class if you're into Classical music. Also have to try their Ristaffel (sp?) meals.

Enjoy Amsterdam. One of the great cities.

EDIT. Just seen binmans post and it's located in that area.