If there WASN'T a Francis Marion site down there somewhere,I'd be VERY surprised.

Both King's Mtn and Cowpens have decent park sites.

Also, I'd look into Camden. Not a park service (is private operated), but pretty good.

A small, but interesting one is the "Star Fort" in Ninety-Six, SC. It was a "frontier" fort, which changed hands between the rebels and loyalists a few times. It is also a good early demonstation of zig-zag trenches in seige warfare.

Remeber, ALL of these are a WAYS from Myrtle. If you were staying in the upstate, I could suggest a NUMBER of sites. One "new" (park JUST opened in last year or 2) one is "Musgrove Mill," which was the large skirmish near where I live. In it, patriots turned the troops (from the 96 star fort) that were going to reinforce Ferguson on King's Mtn. The local claim is that Ferguson was delayed leaving the mtn, as he was waiting on these reinforcements to arrive. Thus, Musgrove Miil shaped the outcome of King's Mtn.

How many and how far you want to go is largely up to you.

While in Charleston, I'd visit the Yorktown (an Essex Class aircraft carrier as a museum since the mid-70s) and Fort Sullivan on Sullivan's Island (what is there now is a Civil War era fort).
Also, I'd stop by and eat at the Queen Anne's Revenge on Daniel's Island. It is decent food for $$$ balance, PLUS the walls are lined with authentic artifacts from the Golden Age of Piracy (included my favorite-> Blunderbusses!!).