Collect a t-shirt from each camper the day before your activity.
Put the shirt in a Ziploc bag with the person’s on it. Do not tell anyone about the game in advance!
When you have all the t-shirts, fill the bag with water and freeze them solid.
Bring out the frozen bags at your activity and pass back out to the owner.
The first person to get their frozen t-shirt out and put it on is the winner.
This is a great surprise when you bring out the bags and pass them out.