You are probably on this already, but I would recommend hiring bikes in DC, great way to get around to see the monuments and they are available from those on street rental stations. It's a really nice city, though it will be very hot and sticky.
Sb is right about it being "hot and sticky" but because of that the bikes are not a good idea. The hop on, hop off bus is the way to go, stops at all the places you'll want to see.