Pretty simple, but fun!
Take two or three of your folks and give them cans of Silly String. Have another group put on garbage bags. (Cut holes for the arms and head first!) Put the two groups on opposite sides of the youth room, with one specific Bag Person associated to a specific Can Person. You should have three or so "one vs. one" setups (hopefully, with lots of innocent bystanders in the middle, watching), then the players say, "Go!" The goal is for the ones with the cans of Silly String to try to cover their corresponding bag people with silly string. The bag person just tries not to get sprayed.
There are two winners: the one who sprayed the most string onto his or her target Bag Person and the Bag Person covered by the least amount of string. A lot of the fun of this game isn't just the spraying that hits the right people, but the frantic off-target shots of string that hit everyone watching as the ones wearing bags are running around. The only thing to remember is there will be some cleanup, and you have to be willing to speak to a group of teens in a room covered in strings. But you knew you would have to do that every now and then when you signed up for youth ministry, right? Right.
Remember Mr. and Ms. Youth Pastor, in some places possession of Silly String is prohibited because it is, of course, a very dangerous weapon in the wrong hands. Please consult your local ordinances and respond accordingly.
Originally published as "String Assassin" on Tips for Teaching Teens. Reprinted by permission.