It is really depends on your time and interests
Area to visit:
National Mall, Washington Monument,
US Capitol(lined up),
White house (only outside view)
Franklin Delano Roosevelt Memorial,
Jefferson Memorial, Lincoln Memorial,
the World War II Memorial,
Vietnam Veterans Memorial,
Korean War Memorial etc..
(Note: walk through will be enough, take some pictures that's all, and also all those memorial are open day and night)
There are 15 museum under the Smithsonian Istitition (such as Air and Space museum, Natural History meseum, Hirshhorn Museum and Sculpture garden) you need to pick up the ones you are interested.
There are also other museums such as National Gallery of Art etc..
You need both Day 1 and Day 2 to walk through the mall, suggest to pick left/or right side of the Mall and keep on the same side, so that you would not cross the mall all the time.
Day 3: The Basilica of the National Shrine of the Immaculate Conception is a prominent Latin Rite Catholic basilica located in Washington, D.C., honoring the Blessed Virgin Mary as Our Lady of the Immaculate Conception, the Patroness of the United States. It is the largest Catholic church in the United States, the eighth largest religious structure in the world, and the tallest building in Washington, D.C
After that you can drive to Baltimore harbour, take a tour boat, visit the Aquarium, eat some crab.
On your way back, you can stop by Philadelphia (optional)
One more thing is very very important, when you eat/touring ChinaTown, make sure you do not go farther but come back toward the Mall side, not the other direction, the oposite side streets after ChinaTown are realy bad, just like some bad area in Chicago/Detroit.