rope
plastic tarp
6 or so balloons

String the rope between two trees or two posts - at least a foot higher than the tallest scouts.
Drape the plastic tarp over the rope so teams can not see each other.
Mark court boundaries on the ground.

Divide into two teams.
Object is to make the balloon touch the ground on the other team's side.

Start a balloon on one side of the court.
A team can hit the balloon as many times as they want before hitting it over the net.

Add another balloon after scouts get the hang of it. Add more balloons if the skill level warrants them.