The 9/11 memorial at the Pentagon is really well done. My wife's family lost friends who were on that plane. It's really powerful. Obviously, kind of a buzz kill.
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Holocaust museum
Museum of the Bible
The Smithsonian (air and space as well as natural history)
There is definitely more to see than you can take in within a weekend. Hopefully the new administration with take down all the stupid fencing put up after 1/6. They made the place look like a prison camp.
Chinatown if right by the National Mall and has some great spots. Most food will be better out toward Embassy Row if you like international or near Adams Morgan. Most places near the National Mall are spendy.
First on my list
Take the DC tour at night! If you don't want to do a guided tour, then at least walk around the national mall area at night.
The Wharf --- Hillwood --- Smithsonian --- Air and Space -- Natural History
venture out -- Jefferson's (Monticello and Poplar Forest) Ok, it's a drive -- Fredericksburg and Shenandoah parks--several little canyons and waterfalls (just look them up), Harper's Ferry (there is a lot to do here besides what you think) -- CEB Tower -- you may want to go up to Mt. Vernon, lot's to do around Alexandria for the day.
(1) Take time to walk the National Mall, you'll see the Vietnam Memorial, Lincoln Memorial, and several other A-list sites. And if you still have time, you can always pop into one of the many museums.
In addition to the many suggestions, add Arlington and the Tomb of the Unknowns then each of the War Memorials are very moving, I'd suggest stopping by them all.
Add the National Archives to see the Declaration of Independence, Constitution, and Bill of Rights.
Contact your Senator and get a tour of the Capitol, depending on timeline of your visit.
We walked some of the monuments/memorials at night and it was really cool. Way less people and the lighting gave it a whole other feel. The Holocaust Museum was something else. If you are into Native American history the National Museum of the American Indian was cool. The Smithsonian, obviously. You can also call your Congressperson's office and try and arrange a private tour of the Capitol. We toured with an aid and just the 2 of us and it was nice. Got to go behind some doors you dont see with a normal group tour.