You spend more than a day just walking up and down the mall look at various smaller museums. The Smithsonian Renwick Gallery was good. I thought the modern art gallery was a waste. The aerospace is a little ways out of town and there is more than enough on the mall to fill your time. I also liked the Bureau of Engraving and Printing.
Wife wants to do the Bureau of Engraving and Printing .... she hears they’re pretty busy making money over there.....
You need to get tickets, free, to B of Engraving & Printing, Washington Monument (if they are back in operation after earthquake) and the Holocaust Museum. They run out of tickets fairly early for all three, get them early and then do something else until your allotted time.