Wanted to go to the Building Museum for quite some time, and had the opportunity when my parents were in town. Great lines, and stunning architecture. Take the tour, which gets you on to the 4th floor which is otherwise closed to the public.
National Building Museum, Washington DC
http://www.flickr.com/photos/unclevanya/sets/…5479/
http://www.flickr.com/photos/unclevanya/sets/…5479/