Agnes Scott College
The college is only 3 blocks from the Decatur MARTA stop, and the cost is only 2.50 per ride. Take any train leaving the airport and get off at the Five Points stop. Change to the Blue Line going East and get off at the Decatur stop. Here is a link to the MARTA map.
If you need to park, you can park at the Marriott (if you're staying there). Otherwise, visitor parking is on the "Main Loop" in front of the campus on East College Avenue (P1 on the campus map) and in the West Parking facility on South McDonough Street (P2 on the campus map).