Tell the class that you have scanned the ground and have dubbed a specific piece of garbage the "magic" piece of garbage. Whoever picks up the magic piece of garbage gets a prize.
You stand by the garbage pail and each student shows you their garbage as they toss it into to bin. Let them know you won't tell them who has won until the floor is clean. Give them a specific start time (ex. they can't start until you say "go!"). This works with all age groups, and really gets the room clean in a hurry!