1) The
MarshWalk and Veterans Pier behind the restaurants in Murrells Inlet. It's free and a great place to stroll while overlooking the marsh in Murrells Inlet. In the evenings, most restaurants will have live music which you can easily enjoy from the boardwalk. Very family friendly.
2)
Huntington Beach State Park, just South of Murrells Inlet. I think a day pass is $5/person. The north beach in the park is by far the best beach in the vicinity. Both beaches in the park have bath houses. They also have an education center which most kids enjoy. Feeding time is 11am and worth seeing.
3)
Brookgreen Gardens, across from Huntington St Pk. Pricey but a ticket is good for multiple days. Magnificent gardens and beautiful sculpture.
Enjoy!