Another quick note about Washington DC - they have a Metro that goes all around the city. (Think subway though some of it is above ground) It is a great way to get around DC with maps available so you can plan a little.
I agree with all about making Lancaster part of your trip! The drive is worth it for the sceanery alone and once you you arrive - well, just wear comfortable shoes, loose waisted clothes(room for eating), and enjoy. :-)