If anybody ever visits Washington D.C., it's very easy to get overwhelmed with the world class Smithsonian and National Gallery museums (amongst the dozens of others). But I highly recommend taking the time to go to Baltimore and visiting this museum as well as the nearby Walters Art Museum, both also very very good museums.
Other great places for awesome museums within a day-drive of D.C.:
Richmond, VA - Virginia Museum of Fine Arts
Dulles, VA - Udvar-Hazy Air & Space Museum
Various National Battlefields sprinkled all over the area
A bit further out, but possible in a day and easily world class, the Philadelphia Museum of Art
Other great places for awesome museums within a day-drive of D.C.:
Richmond, VA - Virginia Museum of Fine Arts
Dulles, VA - Udvar-Hazy Air & Space Museum
Various National Battlefields sprinkled all over the area
A bit further out, but possible in a day and easily world class, the Philadelphia Museum of Art