Visit a book store like Politics and Prose or Kramer’s and get some summer reading.
Go through the list of museums on the National Mall and pick favorites to visit—re-visit.
National Harbor—see the awakening sculpture, ride the ferris wheel get some lunch, possibly paddle boat, or ferry
Baltimore Aquarium
DC walking, bus, trolley, boat tour
Holocaust Museum
Udvar Hazy
Movies or theatre performances
Take in a baseball game—pro or minor league
Paint your own—pottery
pick your own berries
Historical house tour:
http://dchousemuseums.org
Archeology—Alexandria Museum; dino park in PG
Goddard Space Flight Center—rocket launches
Historic farms, walking tours
Cryptology Museum
White Oak Duck pin bowling
Check your local library calendar—they list events by age group.