J.Y.
Hi J.,
Make sure you judge the event by highest percentage of body weight lost. Record everyone's starting weight, and have them weigh in weekly. Instead of reporting everyone's weight, you could report on the % of body weight lost.
The other thing you could do is to do a true "Fitness" challenge instead of weight loss. Have everyone do a short workout, and then in 60 days repeat the workout and see who improves their time. We do this at our gym. Here is the short workout we use, and it's done for time:
Row 500 meters
40 Air Squats
30 sit-ups
20 push-ups
10 pull-ups
You could also do something easier such as seeing who can improve their 1 mile run time the most.
Just a thought! Good luck!
J.