National Air & Space, Museum of Natural History, Library of Congress, Lincoln and Jefferson Memorials, Washington Monument (check to see if it's closed for tours still), White House, Capital, Holocaust Museum, on and on. So many things to see. Some things require advance preparation and tickets.
How long you stay will dictate your itinerary. We went again 2 years ago for a week and with a well planned daily schedule still didn't get to everything on our list. When you go will dictate the kind of crowds you'll encounter.
Enjoy your trip.