One of the great things about growing up in the DC area are the Smithsonian Museums. Excellent in their own right, they're also completely free to visit. (Imagine my shock realizing most museums don't operate this way!) Opened in 2004, this was my first visit (along with my nephew).
While the Smithsonians are very cool, they're mostly the product of a somewhat bland 70's artistic sensibility. The American Indian Museum is different though -- the building itself is as big of a star as the exhibitions are. The exterior in particular incorporates water into its design in really interesting ways.
