I agree with all that has been said... have them compare a "regular" pack to a UL pack, maybe lay the contents out side-by-side so they can see that many of the contents serve the same purpose, just weigh less. Show them how quickly things can add up by doing a side-by-side comparison of the top 3 items, clothing choices, tools/utensils etc. Have boys bring their top "must have" item for the weigh-in and to see if there is an alternative they can consider.

Sounds like fun, but tough to fit into 30-45 minutes!

MNS