The Smithsonian Museums

While in Washington DC, a must see is the Smithsonian's museum collection. Some of the most popular include the National Museum of American History, The National Air and Space Museum and The National Museum of Natural History, along with numerous art museums and the national zoo.
The National Museum of American History is, in my opinion, the most interesting out of the three. If you are interested in the American military, the United States transportation history, or even the history of birth control in America, this is the place to visit. It contains some of the first forms of currency used here in the United States, parts of the Berlin Wall, an authentic Ford Model-T, and even parts of the World Trade Center from September 11th.
The National Air and Space Museum is for anyone that enjoys learning about space travel and airplanes. This museum contains information on the beginning of commercial air travel, military planes from World War I, Amelia Earheart's airplane, and Apollo 11's command module "Columbia".
The National Museum of Natural History is great for children and adults alike. It houses the fossils of a T-Rex, the Hope Diamond, a great mammal exhibit and an I-Max theatre.