The Air and Space Museum was very interesting, and we stopped by to take a bunch of pictures of the different aircraft to show our dads when we got home. The museum also taught a lot about the history of the wars. We stopped by the Smithsonian Art Museum (the one with older art, not modern) but were made a little uncomfortable by the lack of clothing in nearly all the paintings, so we decided to go to the National History museum. We spent hours there, seeing the Hope Diamond, plenty of dinosaur bones, a whole exhibit on sea life, totem poles, and many other treasures.
However, our favorite museum, and the one I definitely most recommend visiting while in Washington, D.C., was the American History Museum. It's full of assorted wonders--Abraham Lincoln's famous hat, Dorothy's ruby slippers, Elvis's guitars, an original Barbie and GI Joe, a collection of the First Ladies' inaugural gowns, the original star-spangled banner, even the puffy shirt from the Seinfeld episode named for it. It was definitely one of the most fascinating things we saw in Washington, D.C., and a must-see, because there's something that everyone can appreciate there.