You can't go wrong spending the day around the National Mall, viewing the Washington Monument and other memorials (Lincoln, Jefferson, WWII, Vietman, MLK Jr., etc).
A recent addition to the Smithsonian national museums, and a MUST SEE. It can be very heavy, but still inspiring. It is open now, and requires free timed-entry passes that you can book online.
You can see the Wright Brothers plane and the Lunar Module from Apollo 11, for starters. It's great for kids and adults alike! It reopens on July 30 and you can book your free timed-entry pass starting July 23.
An extension of the Air and Space Museum, near Dulles Airport, if you have the time to make it down to northern Virginia. Open to visitors now, with a reserved timed-entry pass (free).
Free! This is the oldest state house still in legislative use in the country. Great architecture, and the views of the harbor are fantastic.
40-minute tours - hours a bit limited since it's early in the season, but great if you have the time! $17/adult, $6/children 3-11
Where the inspiration for the Star Spangled Banner happened
A must-see for first time Baltimore visitors
The ceremony and reception are in a building separate from the main museum, so you may want to visit the full museum!
Tours aren't available but their taproom is open and their IPAs are excellent.
Another site for beer lovers.