MARC runs with less frequency on the weekends. If you just miss one you'll be cooling your heels at BWI station for some time, waiting to go to DC.
Then again it takes quite awhile to collect a rental car at BWI owing to the far-out-of-the-way rental facility. It is miles from the airport.
Then again the B30 form BWI to Greenbelt operates on a low-frequency schedule as well.
I would drive to Greenbelt or New Carrolton and catch Metro, I guess.