This is one of the great small museums in DC. It i...

This is one of the great small museums in DC. It is a private museum, displaying a private art collection, but it is free to the public during the week. It is worth it to pay entrance if you visit on the weekend, and any day you have to pay to see special exhibits, which can be fantastic.
This collection has an amazing selection of very recognizable and famous paintings by a variety of artists, not limited to one niche or genre. Among my favorite pieces are paintings by Cezanne, Pissarro, Rothko, Mondiran, and Miro. You have to see the piece called "Wax Room" by Wolfgang Laib in person.
If you live in DC, this is a great museum to be a member of, there are frequent member only events, and you can bring your out of town guests in for free.
The cafe is a great place to take a break and get a coffee or even lunch, on a nice day the patio is a wonderful spot to relax. Last but not least, you'll find something for everyone in the gift shop.