I would recomend Billy Dreher Island State Park, it is on Lake Murray about 10 to 15 miles off of I-26, there are 2 camping areas on the island I like the camping area on the back of the island. The sites are easier to get into, they do not have full hook up, just electric and water, they do have 2 dump stations. I have not been yet this year but last years rate were around $20 a night.
If you are just passing through and need a place to stop, we live out in the country about three miles off of I-26, we are about 40 miles from Columbia. You are welcome to stop there if you would like.