The Hirschorn Museum is a beautiful building with a fascinating collection of sculpture. From some of the lobbies you can see across the Mall to the history and natural history museums. Some of my favorites ---
"Moonmad" by Max Ernst
Arthur Dove, "City Moon"
Calder -- I touched one of his mobiles and it moved
"Still Life: King of Diamonds" by Leger I don't remember ever seeing this one.
"Tropes de Teens" by Stuart Davis
Len with a Miro
Conceptual: Joseph Kosuth's work IS its name: "Four colors four words"