The National Museum of Natural History is a great place to visit--it has hours of interesting exhibits about evolution, rocks and minerals, dinosaurs, and other fossils. A huge perk--like all of the other Smithsonian museums, admission is completely free. It's a great place to take your kids--but also a great place to visit on your own--it will always hold a special place in my heart, because I went there on my first date with my fiance. They also have some interesting (sometimes unrelated) exhibits that change from time to time--for example, when we visited that time, they had an exhibit on the history of baseball.
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