We both grew up in DC and lived there as adults for many years, too. A recent addition which we visited in 2014 is the the very moving Pentagon 9/11 Memorial to those who perished on the plane and in the Pentagon itself. It is outdoors and not far from the Pentagon City shopping complex and Metro station.
I worked at the John F Kennedy Center for the Performing Arts when the Millennium Stage was started. Free performances are given daily at 6 p.m. In the Grand Foyer, so that's a good time to visit the building, which is a Presidential Memorial. Nice views from the Roof Terrace, too.