American Airlines have round trip airfares travelling to and from Atlanta (ATL) and Philadelphia (PHL) for $93, nonstop. This is American's basic economy fare, which limits carry-on bags to one small personal item, such as a purse or laptop bag. All other baggage must be checked for a fee.
You also have the option to upgrade to a main cabin, regular economy at checkout. The round trip price for that would be $180 per person.
Departs ATL: Feb 14, 17, 20, 21, 24, 28
Departs PHL: Feb 13, 14, 17, 21, 24, 27, 28, March 7
Use dates and book it on Orbitz [orbitz.com]
or American Airlines [aa.com]
directly. Bundling hotel with airfare will save money.