We have lots of water toys, floats, kayaks and an island for them to play on. They ride their bikes and fish. My son also plays basketball and they usually make friends for the week. I also let them bring a friend on our shorter trips.
A fun game we play is walkie talkie tag. We use GMRS radios, everyone goes and hides, entire campground is the playing field. The hiders then give clues to their whereabouts. The kids are also require to carry the radios at all times when they are away from camp.
If they get bored I make them walk the dogs and that usually puts a stop to that.