Underrated are the Renwick, Postal Museum, and Udvar-Hazy. Hirshhorn is also great but I find it's more hit-or-miss based on the active exhibit. A few years ago I made a point not to miss Yayoi Kusama (see attached pic for what one of the little rooms was like).
Renwick is a small art gallery located immediately NW of the White House. The Postal Museum is over by Union Station. Hirshhorn is on the south side of the mall. Udvar-Hazy houses all the larger aircraft out in Chantilly near Dulles (IAD).
Pre-pandemic you still needed tickets for the African American and could be difficult at times. They're all worth it so keep trying!