If You Have Only One Day
One day is barely enough to get a feel for the city and see what it has to offer, but if you are in town only for one day, head straight to the National Mall and visit the Smithsonian museums that interest you and your family the most. Plan on eating in one of the cafeterias or museum food courts, and preorder a ticket for the Washington Monument online, so you can make sure to see the city from this vantage point and experience this grand architectural and historical monument.
Washington's art galleries have banded together to hold mutual openings so that you may see several new works in one night. In the Dupont Circle area, on the first Friday of every month, the galleries along Connecticut Avenue hold opening-night receptions. The galleries along Georgetown's main drag hold their opening night on the third Friday of every month.
If you have some time in the evening, race down to Union Station, or check with your hotel concierge to find out if you can take a nighttime tour of the city's monuments (by bus, Tourmobile, trolley, or van), which should include the Lincoln, Jefferson, Vietnam Veterans, Korean War, and Franklin Delano Roosevelt Memorials.