The Washington, Lincoln and Jefferson memorials get all the attention but the Roosevelt memorial was perhaps the most interesting - it is split into 4 areas, each focusing on one of his terms as president.
The the National Air and Space Museum (NASM) is impressive if you like that stuff.
Udvar-Hazy is huge but a bit of a drive.
Arlington National cemetery is worth going to. Lots of heroes.
Just north of Arlington is the Marine Memorial. You may not have a bus-load of vets photobomb the shot...I was killing a bit of time and watched a Marine officer's retirement ceremony here.
Of course you have the Vietnam wall and other war monuments near it.
I think there is a tour at the Washington cathedral.
Union station.
There must be other stuff, like this...