The Sculpture Garden is part of the National Gallery of Art. It's located at the corner of Constitution Ave NW and 7th St NW. There's a variety of interesting spots including murals, large sculptures, and interesting hidden green spaces. It's open from 10am to 5pm daily and is easily accessible by foot, car, or metro.
I spent an hour with Amanda and Suzette here and we came away with some unique shots!
Here you can see some more of my favorite DC engagement photo locations.