If you didn't work in a visit to one of Washington's great museums, you can still grab a memento or two at the Smithsonian Museum Store in Concourse B at Dulles.
For patriotic books, clothing, music and just about anything else related to the U-S-of-A, check out America! in Dulles concourses B, C and D.
Grab a drink at Old Dominion Brewing Co. in Concourse B or Vino Volo in Concourse C.