You can't just "go to the Smithsonian" in DC! The Smithsonian actually includes 19 museums + the National Zoo!
Many of the popular museums are on the National Mall but you'll also find museums in Chinatown and Anacostia. Two are even located in New York City!
-Museum of Natural History -Museum of American History -Museum of African American History
Since many of the popular museums are based on history, many forget about the art museums. Don't Miss: -The Hirshhorn -Freer & Sackler Galleries -The Renwick Gallery
You don't even have to go inside a building to enjoy the museums. Don't miss the art at the Hirshhorn Sculpture Garden. The National Gallery of Art Sculpture Garden is also on The Mall but isn't part of the Smithsonian.
You can visit the Air and Space museum on the National Mall (closed through Fall 2022) but many of the big aircrafts are in Chantilly, VA at the Udvar-Hazy Center.