We hosted MTWI at our family retreat camp last summer and planning to do it again. We had about 40 participants from ages 10 to 70+. We broke them into teams and gave them each a list of the challenges (I don't think anybody knew what any of them involved) and they assigned a team member to each challenge. I went to the website and picked games that would be cheap and not too messy. Sometimes we modified them to make them a little easier. We tried to keep it un-physically demanding since we were dealing with all ages. Since you will have all young people you can probably pick harder ones.
Some we did: (and I don't remember all the official names)
Back Flip- where you put pencils on the back of your hand and have to flip them up and catch them all- this one was requested to do again this year
Play It by Ear- we put the pennies in identical yogurt cups (5, 10, 15, etc to 35 or 40 pennies per cup) and they had to line them up least to most by shaking and feeling the weight
One with keeping balloons in the air for a minute- we did it with two balloons instead of three
One where there was a strip of double sided tape at the end of a table and they had to roll marbles from the opposite end and get 5 to stick
One where we had a ball of yarn hanging down from the back of their waist (I think the website said to use a yo-yo) then we had 10 empty pop cans on the floor and they had to knock them down- definitely the most fun to watch!
Penny Hose- another fun one to watch- put a penny in the foot of each leg of a pair of pantyhose. They have to put one arm in each leg and retrieve the penny without helping the other arm and then get their arms back out again. We had five pair so 5 people could do at a time.
Everybody thought it was fantastic and had a blast even when just watching and not playing.
We have been watching for new ideas for this year- I think we will do the one Speed Eraser where you bounce the pencils into cups.
