Any big city really
I would use southwest's search by map option - put in your departure city and preferred dates and it will show you where they fly with the prices for each spot. I love it since sometimes I just don't know where I want to go. I just know i want to go somewhere!
A note: check how they would travel from the airport to the city.
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.
For Chicago, Southwest flies into the west side of the city and public transit is accessible (I stayed at an airport hotel there on my last trip).
For NYC, I personally found the cheaper airports (that SW flies to)...that it was difficult/time consuming to access the city (then again, I only researched it briefly).
For Vegas, its a quick cab ride to the Strip. Hotels are cheap, but food and shows (and gaming) can add up very quickly if they are not disciplined about budget.
Charleston? I did Louisville, KY with a cab ride from the airport (not too far).
If the airport is not too far from the destination, cab rides/uber can be very reasonable especially if things are concentrated into walking distance in the destination.