DC is clean, transportation is easy via the metro. Seeing all the monuments is amazing.
DC is a very flat sprawled city with lots of green space and outdoor things to do.
Rent a bike and bike around the nations capital, kayak on the potomac river. Play flag football on the mall.
Don't try to park downtown, do see all the monuments and museums.
Hit Georgetown or Adams morgan for nightlife. See the National Arboretum - the azealas in the spring are gorgeous.