Ice cream is a great activity for this sort of gathering; we often get ours out for our Dulcimer Club gatherings. I make up the mixture the day before, sans the whipped cream (Joy of Cooking has great recipes). It then becomes an activity for the kids who don't play. They get to whip the cream, whack the bags of ice to crush it, and take turns on the churn. It does require a bit of supervision to set up and monitor, but not too much. And as a reward for their work, the kids get the first servings.