Cut them loose. I choose my sites carefully and then I let them go. 3,5, and 12 years old (infant too, but she stays in camp with me or momma). They run, dig, jump, splash, catch bugs, pee in bushes... you name it.
They LOVE it. They are of course watched pretty closely, but rarely do we interfere with their exploration. There is something magical about watching them play and learn.
EDIT: I almost forgot. Fire and stick to poke it with. My younger boy will be entertained for hours slowly burning up his "poker" stick. Cutting him new ones every 30 minutes does get old.