Don't think anyone mentioned the National Geographic Museum, usually good stuff, even though the photographs are usually computer prints these days:

http://events.nationalgeographic.com...t-exhibitions/

National Building Museum has had some surprisingly good photo exhibitions for the venue, plus its close to where you are already going. But I don't think photos are among the current offers:

http://www.nbm.org/exhibitions-collections/

Ford's Theatre is good if you like history. Haven't been, but the International Spy Museum is very popular; heard good things about the Newseum (which currently has a Neil Leifer exhibit). Neither is cheap. All are close to where you will be.