Depends who your audience is. If it's around middle school or highschool age, do something interesting that will keep your audiences attention.
For example:
A magic trick.
Bake cookies.
Pack a hiking backpack.
Put on ski/hockey/football gear.
Draw.
How to tie a tie.
How to sell something on ebay.
How to jump a hurdle.
Positions for dance/ballet.
A card trick.
A dance routine.
Playing the drums/piano/guitar/violin etc.
How to study for finals.
How to make a demonstration speech.
How to make a speech.
How to survive teacher's lectures.
Getting people's attention.
Etc. You can pick anything and make it last up to 3-4 minutes, just by giving history, giving step by step, and explaining each step for a few seconds.