District of Columbia

Washington, D.C. isn't a state, but it functions like a city-sized destination with more free museums than almost anywhere else in the country, thanks to the Smithsonian's network along the National Mall.

The monuments, from the Lincoln Memorial to the Washington Monument, are all within walking distance of each other and lit for evening visits. The Capitol and White House anchor either end of the same stretch. Neighborhoods like Georgetown and Adams Morgan give the city a life outside government business entirely.

Most of the Smithsonian museums charge no admission at all, which is easy to forget until the bill doesn't show up.