I would recommend Marriott's Grande Ocean resort on the coast of Hilton Head, SC. From Philly, it looks like a 2 hour flight to Savannah, GA (SAV) then a 1 hr car ride onto the island.

Hilton Head Island is a great place to visit if you have never been there. There are beaches on all sides, the particular resort I suggested rents out bicycles which you can use to ride all around the Island on the bike paths if you want to take a break from the beach.