Every year, my mother and I travel to DC to visit my father. We try to visit a new museum every time but a few must-see spots on our lists are:
The International Spy Museum
http://spymuseum.org/
Air and Space Museum
http://www.nasm.si.edu/
Museum of Natural History
http://www.mnh.si.edu/
Botanical Gardens
http://www.usbg.gov/
Arlington (Changing of the guard at the Tomb of the Unknown Soldiers is a MUST see)
Vietnam Memorial
Korean Memorial (when snow is falling at night is a sight to see)
Also, if you have time to make a short drive to get a bite to eat, The Chart House Restaurant in Alexandria is very nice. Link to map:
http://maps.google.com/maps?oe=utf-8...image&resnum=1
Have a great time while you and your wife are there. Travel safe.