Myrtle Beach is great. Very relaxing, and tons to do.
A few of you have mentioned Medieval Times. James (Maidrite) and I went there for our honeymoon. Before the "tournament" the knights rode around giving roses to ladies in the audience. James said, "It's no wonder our knight lost. He missed the most beautiful woman in the audience!"
There are so many great shows down there--we haven't gotten to see many, but we plan to.
One thing your family might like is one of the dolphin cruises. I think the one we went on was in Calabash, NC, just north of Myrtle Beach. Make sure you are dressed in clothes that you don't mind getting wet though! We were completely soaked by the time we finished. We had a ball though!