Divide everyone into teams (depending upon how many people come to your activity). Some of those who attend can be "audience members", but anyone who would like to help cook should be able to.  Just like on the TV show, have your committee choose a secret ingredient around which all the dishes will center.  Make it something with quite a few possibilities (i.e. cheese, hamburger, tomatoes, chicken, etc.)

Provide for the teams some additional basic ingredients - things like seasonings, pasta, flour, oil and a few fruits and vegetables - that would go well with the secret ingredient.  Give the cooking teams thirty to forty five minutes to come up with two dishes using the provided ingredients, and provide appetizers for the audience members to snack on while they're watching.  When everyone is finished, judge and eat!