There's rock creek park. Also, DC has loads of venues for plays, concerts, ballet etc etc so maybe they can check out what's going on at the Kennedy Center or Wooly Mammoth?

Or they can walk around near DuPont circle and check out all the embassies. Some of the buildings have very cool architecure. I think the open embassy days are in may but maybe there might be other events going on.

What are your folks into?

For other outdoorsy stuff in particular, if you have a car, there's the arboretum, which has some good walks. For smaller museums, Dumbarton house and the hillwood house museum- these are both manor houses with fancy gardens.