On the subject of DC airport transfers, it's previously been been suggested that some car rental companies treat BWI-IAD-DCA interchangably, with no drop fees, which might make a one-day car rental a cheap option. Never explored this myself, so don't know if it works.

On the MARC - note that they've increased the service frequency w/in the past few years, so there are some 22 departures daily and this is a better option than it used to be. In your case, however, since the last northbound is at 10:40, it won't work. MARC also continues to run M-F only, too.