Here's a few of my favorites that I've put into different categories for ya. Hope you enjoy!
The mad scientist:
*The effects of gasoline...on fire (my parents rejected my idea)
*The effects of a monster energy drink, a box of candy, and a kazoo on a younger sibling (I got in some real trouble for that one)
*The effects of electricity (30v or more) on your face (I was a pretty violent kid growing up now that I think about it)
The biologist:
* what different common plants look like under a microscope ( you'll need a microscope equipped with a camera for this one)
*the life cycle of frogs (requires tadpoles and the know-how on keeping them alive)
*what lactic acid feels like (requires human test subjects to run until it burns...which is the lactic acid)
*why you need to wash yo filthy hands! (Requires pictures of bacteria)
*different types of fermentation (making salmi and pepperoni will help you win, but just explain how wine is made...don't actually make it...don't ask me how I know this either)
The chemist:
*the wonders of sodium silicate (DO NOT EAT--just look)
* the effects of gasoline on fire (seriously you should consider doing this one...it'll be fun)
*the chemical reactions of acids and bases
*what is "hydrophobia" (requires water and hydrophobic sand)
*how to distill stuff (DO NOT MAKE WHISKEY LIKE I DID...apparently teachers...and police officers do not like that) --but this one can be pretty cool if you can obtain the proper glassware and choose to distill something thats not fermented...in fact, just distill water.
The rich kid (if your that guy):
*3D printing
*EMP generator (don't make a powerful one...just kill a flashlight or a calculator)
Well I sure hope this gives you some ideas...lemme know if you need some more suggestions