Be sure to check out the Washington Monument - great for selfies and drumming on the lights.
The Lincoln Memorial is best viewed from someone's shoulders.
And no trip to D.C. would be complete without a stop at the Floral Library. Yes, the Floral Library. Tulip tasting and sniffing by immersion are required.
One last thing: Be sure to take your family with you. All trips are better that way. (And the big kids highly recommend The Holocaust Museum.)