The Old Post Office, Washington D.C.

Once the city's main post office, it was saved from demolition and is now a popular tourist attraction. Where they once sorted tons of mail on the main floor, there's now an open atrium surrounded by shops and restaurants. Worth a look, if you don't mind having your bags x-rayed on the way in (because, you know, this place has got to be high on the target list of terrorists the world over).