You’ll need a Weetabix (breakfast wheat bar or some other really dry breakfast bar), some undiluted cordial, piece of bread, can of coke and ground covering for floor protection. (Above foods can be substituted for maximum grossness or you could even replace one food with an individual mixture made of different edible’s).
Get up some volunteers (the amount of kids you get up the front, you will need that amount of each of the foods.) Explain to them that they have to eat each of the foods (in order as above) and be the first finished, but they can only pick up and eat each thing one at a time. Buy some bags of lollies or chocolate bars as prizes and reward to the winners.
This is a really gross competition, but it really does attract huge interest. In my high school (12yrs-18yrs old) youth ministry, this had unsaved youth totally captivated and excited with a huge willingness to come again to the outreach events. They love this stuff!