Washington DC

Very easy. Park at Greenbelt station. There is a large Park and Ride lot there. You may want to consider purchasing Smartrip cards for each person ahead of time at the WMATA.com site. That way you're not dealing with the fare kiosks but can just get through the turnstiles and go. Study the maps a bit ahead of time. It helps to know not only which color route you need when connecting but also which direction you need to go, which is displayed on each train based on its final destination. Once you get a feel for it it's a piece of cake. No way would I drive myself around and try to park.

Also, take advantage of the free Circulator buses to get around the National Mall.