You ve gotten some good advice, but I think the easiest, while still not expensive, method is to take the train all the way. You take the free shuttle from the airport to the train station. Arriving on Monday you should be able to get the MARC commuter train. On the return trip, since it s a weekend, you d take Amtrak from Washington Union Station to BWI station (but look at the schedule first, since not every Amtrak train on that route stops at BWI).Here s a link to Amtrak s website:http://www.amtrak.com
Find out how many business days it takes UPS Ground to deliver to Washington from your destination. Then that many days ahead of when you arrive, go to your local UPS store and ship your stuff to yourself care of the hotel. I do it all the time and it s usually cheaper than what the airlines charge to check a bag. And you don t have to worry about lugging it from the airport to the hotel and back.The MARC train runs weekdays from BWI to Union Square where you can catch Metro. On Saturday, the B30 bus leaves Greenbelt Metro Station for the airport.
The B30 Metrobus goes from BWI airport to the Greenbelt station at the end of the Green Line. You would take the Green Line to L Enfant Plaza, and take the Orange Line in the direction of Vienna or the Blue Line in the direction of Franconia Springfield to McPherson Square. It s a bit of a pain, but it s the cheapest way of getting there.
Check the BWI website under Ground Transportation . There may some type of shared ride or shuttle service.