There are two campgrounds at Congaree.  You park at Longleaf parking lot, where there are walk-in sites and vault toilets (clean but terrifying to squat over the abyss). We then walked 1 mile to our campsite at Bluff, where there are 6 sites available.  The sites are situated around a large, grassy area, and there isn't much privacy. (From what I saw, Longleaf sites offer more privacy.)  But you can't beat $5/night.

The walk to Bluff is not difficult but there are lots of tree roots.  We brought a beach cart to help tote our stuff, and we had to navigate those roots.  You will be packing light!

The very nice and air-conditioned Harry Hampton Visitors Center has a water fountain for filling bottles.  You will need lots of water if you are camping during the summer months.  It's also 1 mile to the visitors center.

The trails are well-marked, which is great when you're trying to find your way back to your site in the dark.

There is lots of hiking (easy and difficult) and good fishing in Weston Lake.  Canoeing and kayaking are completely dependent on the water levels at the time, which fluctuate greatly.

There is nothing in the way of swimming, which is a bummer in the summer.