There are a few that you can easily double up on Freer/Sackler (Asian Art) as well as the African Art are right there and smaller than some of the others.

The American Art and Portrait Gallery are in the same building and an easy two-fer. Get lunch/dinner over in China Town or on 7th Ave when you’re there.

Whenever you hit American Indian, get lunch at their cafeteria as they have the best cafe of all the Smithsonians.

The Air and Space on the mall is only 1/3 open right now but they have some stuff there. The bigger air and space museum is the Udvar Hazy center out by Dullas airport, it’s a ways out there but there’s so much it will take a good half day or more.

The zoo is a Smithsonian but on the edge of town. The Renwick Gallery is over by the White House.

Of course all that is less than half the Smithsonian’s, Still have the Postal Museum, Hirshorn, Natural History, , American History, Arts and Industry, African American History and Culture, The Castle, and the Anacostia.

The National Gallery of Art is not technically part of the smithsonian, but most assume it is as it’s an art museum right on the mall next to the others (and if you are interested in art they have a good large collection).

Keep an eye out as Air and Space, National Zoo, and I think African American History and Culture might still be doing timed tickets that you need to reserve in advance.

Pick the ones you want to focus on. If your’e a completionist, you can speed run a lot of the ones on the mall by just poking in and seeing one thing at the spots you’re less interested in so long as they’re not doing timed tickets.