I can't say I'm a big fan of traditional museums, but I nonetheless still enjoyed the National Air and Space Museum for the most part. There are scores of exhibits representing air and space and several historical items (like the spirit of St. Louis). It's been a few years since I've been, but when I went, I do remember getting somewhat bored despite being a fan of this type of stuff. As someone said, this is probably the better of the museums in DC. I would recommend not spending all of your time here but spreading it out among the different landmarks.
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