Greenbelt is closest Metro stop from BWI.
Yes the parking is free on weekends and
federal holidays. I don't know if there
are overnight restrictions for weekends or
federal holdiays. Greenbelt is the green line there are plenty of maps inside the station and on the trains. Purchase a pass for $5 per weekend day and ride from 8am to
approx midnight, just keep in mind when the last train leaves. Leave no valuables in your car or in plain sight, lock the car.
Quick Directions (BW Parkway (Rte. 295) towards Washington and exit Greenbelt Rd. (Rte. 193) stay in the right hand lane and make a right turn before Kenilworth Ave, this turn puts you onto Kenilworth. Once on Kenilworth - move over to the left hand lane as soon as you can and stay in the left lane; three traffic lights make a left by the Fed. Court
House as a landmark there is a large Metro sign. Good Luck and I think it's three lightson Kenilworth they keep adding 'em.