Spring & summer are peak tourist seasons for Washington, DC. Lots of high school trips in Cherry Blossom season, so be patient. Don't try to do too much in 1 day. Take a break now & then, like the movies at the Air & Space museum to get off your feet a bit. The Museums & Monuments on the Mall are free, which adds to the enjoyment.
The National Zoo is worth a visit. Take METRO, it's between 2 stops, so get off at Cleveland Park when arriving, and get on at Woodley Park when leaving - that way your walk (couple of blocks) is down hill both ways.