Visiting all or even most of the Smithsonian museums and galleries might be too much for you, especially if you are in town for only a long weekend. In this case, you might wish to concentrate on the three must-see Smithsonian attractions — the National Air and Space Museum (the only one of its kind in the world), the National Museum of Natural History, and the National Museum of American History. Plan on dedicating at least three hours to fully explore each one of these museums.