Something most tourists overlook: National Auboretum (http://www.usna.usda.gov/). If you're in the Monkey County area, there's Brookside Gardens in Wheaton (http://www.mc-mncppc.org/parks/brookside/)

Another often-overlooked touristy thing is the National Aquarium in the basement of the commerce building in DC(http://sc94.ameslab.gov/TOUR/aquarium.html), which is not to be confused with the "National Aquarium in Baltimore": they're both the "National Aquarium", but the one in B-more is larger and justly gets a lot more attention.

Two of these three are actually in DC, though, and the other is in the 'burbs, so if you're looking to get out of DC, I've steered you wrong.

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