If you get the chance to see Ai Weiwei's 'According to What?' at the Hirshhorn or Roy Lichtenstein: A Retrospective at the National Gallery of Art - take it! We recently spent a relaxing Saturday afternoon wandering around the Mall and seeing these 2 exhibits (and made friends with some VERY friendly squirrels along the way).
My favorite installations had to be the huge bowl of pearls and the giant light cube from the Ai Weiwei exhibit (pictured below). Photography wasn't allowed in the Lichtenstein exhibit, but trust me when I say it's a 'must see'!