For this Spark, challenge your Section to build something that can fire a ball to hit the target you have chosen. Ideas could include; a catapult, ballista, trebuchet etc.
Ball to fire
Target or something to knock over - tins, plastic cups, etc.
You could provide them with instructions to follow or give them free reign to be creative with their ideas.
Once built, they will need to test them to see if they work. If working in teams this could have a competition element to it, with a prize for the winning team.