K: This place is really awesome! An old power plant in Georgetown - you can explore both levels of the plant, with the original pipes and everything. Some more pics: http://www.flickr.com/photos/robotkenshi/tags/powerplant/
Josephine S: Although you can't walk there, it's not in a walkable neighborhood, and it's all about flying -- this museum is a must-see. They have an incredible plane collection including JFK's Air Force One, a Concorde, and our kids love the SR-71 Blackbird.