A favorite author Roald Dahl and Carter's favorite book BFG, Big Friendly Giant. Carter read this several times when he was younger. The National Theater in DC put on a wonderfully adorable and funny production of BFG.
POV restaurant, a great view of many things in DC (White House and Luke more fascinated with the snipers, Washington Monument and many more).
White House Visitor Center (amazingly the boys are still in love with history and actually enjoy these places).