The Spy Museum in DC is a fun way to spend an afternoon. It's full of real-life spy gadgetry like cameras and bugs as well as historical artifacts like a letter from George Washington directing the creation of a spy network (more about George Washington, spymaster). There are also interactive touchscreen displays, such as exhibits that invite you to spot the dead drops and a visualization of the Enigma encryption engine.
The museum starts off with the tools of the trade. Some of my favorites included a beautiful example of hiding in plain sight -- a camera hidden in the side of a camera case -- as well as a camera attached to a pigeon. There was also a great display of various bugging techniques and a replica of a bug so ingenious that I still don't understand it: Russian school kids gave an American ambassador a carved eagle seal with a hidden air cavity and passive antenna, which could be activated as a bug by aiming a particular frequency at it.
The museum then moves you through a wide swath of spy history from historical figures like Sun Tzu, George Washington, and Harriet Tubman, to historical events like World War II and the McCarthy trials. Of course the vast majority of spy history may never be known, but its fun to peruse some of the more notorious cases.
There are side-games to participate in as you journey through the museum. One is keeping your cover, which requires you to memorize an identity and mission. I thought this would be fun until I discovered that it just involved answering questions at a computer screen -- it would have been much more fun if employees were trained to come up to you and ask you questions about your cover; it just isn't scary when you walk up to a computer screen vs. someone walking up to you.
A less official side game is sneaking into "secure" areas, such as a door with a combination lock that opens into a "Top Secret" room -- actually just a facilities room with the air conditioners. This is a more difficult game to play because it requires you to distinguish between allowed secret areas and actual private areas. I had to debate for awhile whether a staircase marked "No Admittance" was truly that, or just another opportunity to test my covert skills (I figured out that it was the stairway to the administrative offices, so I aborted).