Using fruit, and more generally biological material, makes me a bit uneasy because their transformations are so diverse and complex that it's hard to give a simple and correct cause for them like "oxidation".
I feel this compound shouldn't be missing in your fairhttps://en.wikipedia.org/wiki/Tetrakis(dimethylamino)ethylene
it serves in luminescent toys like collars. Just display a few such toys with posters of the molecule and its reaction with air.
Reactions with air that speak to many people:https://en.wikipedia.org/wiki/Drying_oilhttps://en.wikipedia.org/wiki/Linseed_oil
you can varnish a new surface every hour.
A zinc-air battery? Not very spectacular, so you must add big posters with explanations.
If you find a gas heater that ignites thanks to a catalyst when you open the valve, it has some "magic" in it, nice for a fair.
Illustrate the flash point by a compound that burns only with a wick or when hot. Maybe turpentine, well chosen petroleum, or Diesel oil.https://en.wikipedia.org/wiki/Flash_point
add some candles and explanations.
Pour glycerol on potassium permanganate at room temperature, it ignites if the crystals are well chosen.
Sow iron power on a flame to show it burns.
Burn cotton in air. Compare with a telephone book to show the importance of tight air mix.
Burn ordinary and easily recognized sugar (saccharose in usual cube form) in air using a catalyst (ash).
If you access oxygen, show that fires are more intense than in air.
Need much more preparation and are potentially dangerous:
Ignite something like cotton by compressing air (like in a Diesel engine)
Burn flour in an air stream.