The Lake Shark
The lake shark prank can only be played at a camp that features a lake or pond, as even the most gullible camper won't fall for the prank in a crystal-clear pool. While it is not necessary to set up the prank beforehand, creating a legend that your lake contains a shark early in the year, and saving the prank for near the end of summer, can build the effect. To simulate the shark, have a counselor that can hold her breath underwater swim by a dock where children are hanging feet in the water and gently tug on the legs, or swim with a model fin just above the surface of the water.
Scary Stories
A campfire story-night is the ideal situation to play a prank on the children or fellow counselors. Bring a scary costume with you to camp, then when stories are being told around the campfire, share a story that features a villain that matches your scary costume. Have an accomplice emerge from the darkness wearing the scary costume right as you reach the climax, or have the fun yourself by entering into a cabin later that night wearing the costume.
Bombs Away
Bombs away is a fun prank to play on the children as well as on new counselors in their first year. Organize a game where players can find themselves in jail, such as a capture the flag game, and set one jail under a tree with many branches starting at a reachable height. Prior to the game, fill a garbage bag with water balloons, then climb into the tree with the bag, high enough to not be seen without climbing above your comfort zone. Wait for a large group to gather under the tree, either while tagged in the game or while the counselor leading the game is taking the group around to show the jails, then drop water balloons from above. Don't worry if they break on lower branches, that just makes for a wider spread and more hits.
Buckets of Fun
Similar to bombs away, buckets of fun ends up with the marks of your prank all wet, so be sure not to perform it in a setting where the marks could have electronics, like a cell phone, on them. Find a building in the camp with a window over a doorway that doesn't contain a screen and is positioned over a door. Fill a bucket or large bowl up with water and bring it to the window. Have an accomplice lure the mark outside through the door then dump the water from above.