Scatter some cones (divots) across the area, some the right way up, and some upside down. Split the group into two teams - 'Hackers and Greenkeepers' - and spread them out across the area.
On the word 'Go' the Hackers run round turning the divots upside down and the Greenkeepers must turn them the right way up.
After a set period of time, count the divots to see how many are the right way up and how many are upside down.
Change the teams round and start again.
Progression:
Easier: Reduce the distance between the divots.
Harder: Increase the distance between the divots.
Rebounding is the least glamorous thing in basketball and one of the most decisive. It is also almost entirely a habit, which means it is entirely coachable.
Good spacing is only the start. The best modern offences teach players to attack the defence's rotations and play in a permanent 4-on-3 - here is how to coach that read.
The ball screen produces almost half of all professional offensive possessions, yet most teams still teach it as a memorised play. The modern approach trains the read - giving players a framework to decide based on what the defence does, not what the coach called.