The Potato Chip Challenge is an actual (free) national challenge in which your students can compete, but I think it could also work beautifully as part of a week-long (or longer) science camp.
For the challenge, students need to engineer the smallest, lightest package to protect 1 Pringles potato chip in the US mail. The weight and volume are recorded and then the potato chip is sent to someone else participating in the challenge. When the chip arrives at its destination, it is evaluated according to standard criteria.
To use this in a camp setting:
I would have students design and construct their packaging on the first day of camp. I would have them address their packages to themselves, in care of the school (or other location) where camp is being held. When camp was over for the day, I would drive to a neighboring community and put the packages in the mail there. (If you live in a community where all of the mail leaves to be sorted elsewhere, you can just take them to the local post office). Then you wait for the chips to come back to you and see how everyone fared.
I think I might just have to give this a try myself... anyone want to trade chips?