We stayed in Garden City which is south of Myrtle. Definitely plan an evening to walk around Broadway at the Beach. They have all kinds of restaurants and stores, as well as an aquarium. Key West Grill has a perfect mojito. If you don't mind driving a little ways, the Marshwalk at Murrells Inlet has great fresh seafood. Wicked Tuna is pricey but amazing food.
Boca Catalina, Arashi Beach Manchebo Beach, Eagle Beach
I agree with Walt. If you decide not to go to Hilton Head, all of the beaches that he mentioned around Charleston would be good. Then you could also visit magnificent Charleston. Just a heads up, Folly Beach is where all of the college kids hang out, and it can turn into quite the party beach at times. But I still suggest looking into Hilton Head, also. It is one of our favorites. Since it is a 9 hour drive for us, we don't get there very often.