Using ropes and staves, Cubs to build a volleyball net.  They can tie the ropes to trees or use fixings in the Hall - the net doesn't have to be perfect but is an opportunity to practice sheet bends, round turns and two half hitches and reef knots.  Maybe provide two ropes strung up to start and let the Cubs tie ropes in between the two main ropes.  Can use staves as well and create tripods as the stands for the net.  Decide based on time available in your programme.  

Basic Rules:

Volleyball, game played by two teams, usually of six players on a side, in which the players use their hands to bat a ball back and forth over a high net, trying to make the ball touch the court within the opponents’ playing area before it can be returned. To prevent this a player on the opposing team bats the ball up and toward a teammate before it touches the court surface—that teammate may then volley it back across the net or bat it to a third teammate who volleys it across the net. A team is allowed only three touches of the ball before it must be returned over the net.

The game is played on a smooth-surfaced court 9 metres wide by 18 metres long, divided by a centre line into two equal areas, one of which is selected by or assigned to each of the two competing teams.  You can define your court size though based on the space available and number of players.  With Cubs, keep the area a little smaller.  A tightly stretched net is placed across the court exactly above the middle of the centre line; official net heights (measured from the top edge of the net to the playing surface—in the middle of the court) are 2.4 metres for men and 2.2 metres for women.  For Cubs, it should be just above their heads so they can jump a little to spike the ball.  Serving normally happens from just outside of the court at the back.  

Informally, any number can play volleyball. In competition each team consists of six players, three of whom take the forward positions in a row close to and facing the net, the other three playing the back court. Play is started when the right back of the serving team steps outside his end line into the serving area and bats the ball with a hand, fist, or arm over the net into the opponents’ half of the court. The opponents receive the ball and return it across the net in a series of not more than three contacts with the ball. This must be done without any player catching or holding the ball while it is in play and without any player touching the net or entering the opponents’ court area. The ball must not touch the floor, and a player may not touch the ball twice in succession. A player continues to serve until his team makes an error, commits a foul, or completes the game. Either team can score, with points being awarded for successfully hitting the ball onto the opposing side’s half of the court, as well as when the opposing side commits errors or fouls, such as hitting the ball out of bounds, failing to return the ball, contacting the ball more than three times before returning it, etc. Only one point at a time is scored for a successful play. A game is won by the team that first scores 25 points.