On the free aspect, there was plenty to climb on for the kids; on the money side, they have an inclusive snack bar, an extensive gift shop, a Vietnam Support Base, a Cold War Memorial, a Medal of Honor Museum, and the USS Yorktown, USS Laffey, and USS Clamagore to tour. It was pretty busy so I would recommend buying tickets online ahead of time or you could be shut out, especially on the days surrounding the 4th. When we were there for our ferry ride to Fort Sumter, we saw a bunch of boy scouts heading up Yorktown's ramp for an overnight camping trip.
Some of the free stuff:
Fireworks are launched from a barge on the river, and you can buy tickets for seats on the Yorktown's flight deck. Get there early and enjoying catering, bounce houses, and a band - they sell out early, so plan for next year. Otherwise it's free to watch from Patriot's Point, but you'll have to get there waaaaay early and park waaaay out unless you want to pay $10 to park at the point itself. Decent food from vendors and nearby restaurants was available to kill the time waiting for it to get dark. We parked a mile away and when it was over, we plugged a movie for the kids into the van's player and just waited for the traffic to thin out. If you have a boat you can park in the harbor to watch. The show was fantastic, though, and worth the effort.
Oh, but it is South Carolina, so don't forget your bug spray!