I'm based at KGAI - Gaithersburg, MD. They (airport people) - I believe - can arrange for a car rental (Enterprise?). Also, KGAI is 3-4 miles from the DC Metro (subway) - Red Line / Shady Grove stop, and a taxi can be called to go from / to airport. Use the 'metro' to get into DC, etc. There are stops at most 'touristy places'. Check out Washington, DC metro on the internet for stops, etc. Some people chose to stay in "the 'burbs' " and use metro for $$$ reasons. Several 'chain hotels' along Shady Grove Rd. in Rockville, MD (mile or so from metro).
The GAI airport is 'under new management', and fuel is from a truck that will come to your plane. Apt. # 301.977.5200 - ask them about the above. Their tie down fee used to be $10 per night.
About Freeway - W00 - they are a Mooney Service Center. I have no idea about ground transportation from there into DC. Great place if you have 'an issue' with your plane while in this area. 301.262.6544 is their shop number.
If coming VFR, note the prior comments about the 'SFRA' airspace. The military folks can get rather testy - and the occupants make the local TV news - when a GA plane wanders into the SFRA without permission. This is for IN AND OUT.
Dwight '82J