The National Gallery of art is located on the National Mall. The art is housed in two huge buildings and a sculpture garden, which are all connected by foot (all of which are, also, free). The art is mostly American and European (with Asian art at the nearby Freer and Sackler galleries). You could lose days to the permanent collections alone.
Though there are favorite pieces at other American museums, this one is a vast national feat boasting America's great collections of work throughout history. West Building houses works by Europeans from the medieval through the late 19th century, as well as pre-Modernist works by Americans. East Building holds modern and contemporary works.
The moving walkway from West to East buildings is a stunning way to change consciousness when switching from old to contemporary.
Mon-Sa: 10:00-17:00
Su: 11:00-18:00