Have everyone partner up. This can work better if partners are relatively close in size, but they don’t have to be exactly the same.
They stand facing each other, feet together, arms up like they are going to “high ten” each other. They should be close enough to do this without their arms being completely outstretched when their hands meet.
Now, they match up their hands, and try to knock the other one off balance by pushing on them in quick slapping motions. You can fake a push, or duck your own hands away to try and get your partner to fall off balance.
Any movement of the feet is considered being knocked off balance.