If you are into US history, when you come out of PA stop in Gettysburg. Purchase the car tour CD at the gift shop and you can drive through the park, stopping, or not, at all the sights. We have gone with the kids and stopped so they could climb on things and then gone as far as they could pay attention.
When in DC try to hit the Supreme Court and the Library of Congress and take their tours. They are right next to each other and across the street from the Capital. Amazing architecture!
Check out the www.Kennedy-center.org for tickets. *I think* Wicked is playing around the time you are here :-D
Have a fantastic time. It sounds amazing!