When you visit the DAR Musuem don't forget the "Period Rooms." They are part of the museum, but are scattered throughout the complex. I'm told there is a map of where they all are, but I've never seen one. The state society's have adopted them and are responsible for their upkeep. They're really neat.
Also, if you think you might want to do any research in the DAR Library, be sure you take your membership card with you.
Don't miss the flag that inspired Francis Scott Key to pen the Star-Spangled Banner. It's at the Smithsonian Museum of American History.
Time permitting, Mount Vernon isn't too far away. I can highly recommend a boat cruise down the Potomac to George and Martha's. The one we did included admission to Mount Vernon (tour the house first thing to avoid really long lines).
WDC is one of my favorite places. I hope you enjoy your trip.