Renwick Gallery of the Smithsonian American Art Museum, just a block from the White House. The gallery features decorative arts and is contains many interesting pieces.
The building itself is a great example of Second Empire architecture intended to mimic the French/Louvre style. It was the first American building erected expressly as an art museum. Jackie Kennedy saved it from the wrecking ball in 1965 and the recent extensive renovations and updates have created a great way to spend a few hours.