My son and girlfriend have southwest credits that expire next may. they are under 25 so if they rent a car they would pay a big price. They are on a budget. What is a good vacation spot that you do not need a car rental other than disney.
BWI (Baltimore) is super easy and cheap to DC via MARC commuter train-about 25 minutes and $7 each way straight into Union Station. Make sure to arrive/depart during hours that MARC runs.
For DC if you fly into Reagan, they can take the subway easily. But if you fly into Baltimore Washington (I do because it is much cheaper), it's a bus/subway combo or $50 one way uber ride.