The American history museum is very good and accessible - no boring bits! The original Betsy Ross flag is in there.
Air and Space, well, only if you’re into that sort of thing.
Take a walk by the White House - see both views, front and rear lawn - the rear is somehow more impressive. Take in the Washington monument then head over to the 2nd world war memorial - it’s really well done. The reflecting pool should be filled (on my last visit it wasn’t). See the Korean and Vietnam war monuments - both incredibly moving but very different.
A major highlight is the Lincoln memorial - a stone figure of him is inside the monument and the size takes your breath away!
The Jefferson memorial down on the water is smaller and prettier but no less interesting. The cherry blossom will have gone but the trees and riverside views are very beautiful.
Take a tour bus! I always advise that...but beware of the heavy hints for tips! One of the routes takes you up through Georgetown to gawk at the embassies and big houses.
Arlington cemetery - this requires a full afternoon really or a day for many people as there’s a lot to take in so might not be something for a whistle stop visit.
There’s also the natural history museum if you like to see stuffed animals. The best art is at the Phillips gallery.
But yes, the American History museum is a great choice. Then just walk, walk, walk. It’s such a great walking city and so manageable and historic - I love it.