Bring up 5 or 6 counselors that have no idea what they are getting into. Have them get down on all fours on the ground and place a large cup of ice cold water on their back as if they were a table.
Have 2 campers sit next to each counselor and just begin to talk to each other. Then, have the rest of the presentors run on with sticks or some other non harmful weapon.
They all begin to fight and knock all of the cups of water off and spilling them on the counselor's backs. At the end, the attackers run off and the diners apologize for the wettness and let them sit down again.
This is great if done by the same age group every year because the returning counselors know what is happening and they laugh a lot at the new counselors that have no idea what is going to happen.