Just went to the Spy Museum for the first time a few weeks ago. Its pretty nice, but they set it up in the beginning like its going to be real nice like you've got a tour guide and all ... and after that, its just like any other museum. They have some interesting stuff in there, don't get me wrong ... but I was kinda disappointed after they built it up real big in the beginning. Would definitely check that out if you're in DC. The Mall area is nice too, bunch of museums over there.
The new air and space museum is cool, but like Linflas said, its hard to get to if you're in DC w/o taking a taxi or a bus (Metro won't get you out that far).
If you're looking for a nice place to drink, Georgetown is pretty nice. Sitting out on the waterfront is really cool assuming the weather is nice. Not much site-seeing to be done there though.