A museum in the Smithsonian Institution, located in Washington, D.C.. It's about American history (surprise, surprise). There's assorted historical artifacts, the stereotypical example being the Ruby Slippers from The Wizard of Oz. They have the original Star Spangled Banner that was flying over Fort McHenry when Francis Scott Key wrote the national anthem. Wandering about, you'll come across a random display case containing... something from America's past.
It also has some decent exhibits. I think the first floor is the best. The Information Age was the coolest thing I'd seen when I visited it as a kid. It's getting a little dated, but it's still amusing. It traces the history of computers from punch-cards and ENIAC to the present day. They have all sorts of cool vehicles: huge locomotives, Mac trucks, Model T's, bycicles. They have Mario Andretti's car and a Conestoga wagon.
The second floor is more business-like. After the Revolution has some artifacts from everyday American life in the early 1800's. First Ladies has a bunch of inaugural gowns and White House silverware. It's a nice place to take the class on a field trip.
The third floor gets interesting again. In my experience, it's the least visited. It has the World War II and war in general exhibits. There's the history of submarines, guns, and the army. There's also an in-depth look at the Japanese-American Internment Camps during World War II, which is well worth a look.
All in all, a fun museum, where you could easily spend a few days.
More info at http://www.si.edu