Supplies: Large, black trash bag filled with random items for every team.
Get some large trash bags together (preferably black ones that hide the contents of the bag). Have one for each team of 4-5 kids. Fill the bags with a half dozen of the most random items you can find.
Give each group a bag and tell them they have ten minutes to create a game from the contents of the bag.
Get the groups back together and have them "play" the game for everyone else to demostrate what they created.
The wackier the items, the better. You'll be amazed at what they can come up with.
NOTE: It is not necessary that every team has the same items in their bags. However, if they are the same, it makes the judging easier.
Idea by Cade Nethercott