The 9/11 Memorial at the Pentagon is a must-see. Especially at night. It's open 24/7. (see pic below from a late-night visit I did with family, we were the only people there at 10:00p)

http://www.whs.mil/memorial/


The Newseum gets rave reviews from my guests. Admission fee.

http://www.newseum.org/


Second the Spy Museum - check to see if you should get an advance ticket.


The Capitol Visitor Center (opened December 2008). See what a billion bucks can buy to secure Congress in the event of attack, showcase artifacts and provide extra office space. It's a museum in itself, very impressive.

http://www.visitthecapitol.gov/


What's new at the Smithsonian Museums:

http://www.si.edu/


If you're in to model trains, the U.S. Botanical Gardens has a great display every Christmas. This year's starts November 26. The Botanical Gardens -- adjacent to the Capitol Reflecting Pool -- underwent a major renovation a few years ago and is well worth visiting if on the Hill. Especially if you've any interest in plants - indoor and out.

http://www.usbg.gov/education/events/Holiday-Magic.cfm


You may also want to check out the new (2006) U.S. Air Force Memorial, which is near the Pentagon (1/2 mile walk).

http://www.airforcememorial.org/