You could make hydrogen and oxygen from water. Make a "U" shaped container from PVC pipe. Drop a bare wire from each pole of a 6 volt lantern battery in each side. It will immediately begin to bubble. Hydrogen out one side, oxygen out the other. Capture the hydrogen in a plastic garbage bag and it will float. Later, after the fair is over, seal the end with tape and attach a self addressed, stamped post card to it with a request to note the location, date and time where it was found. You might be surprised how far it will go. Be sure to lable the bag with warnings that it is FLAMMABLE HYDROGEN.
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Oh and don't forget... Hydrogen blows up real good!
...only when you mix it with oxygen....
make a wooden boomerang. boomerangs have a complex amount of physics that make them work. very simple to make and very fun to play.
Personally, I would go for antimatter. If that doesn't fit your taste, I would go for something along the lines of using a spring. Something maybe similar to a mouse trap car, but more original. The math is fairly easy to prove how the energy is transfered from potential to kinetic. Good luck.
The easiest way for me to answer this one is to shamelessly link to one of my previous answers, here:https://www.instructables.com/answers/Epic-Science-Fair-Ideas/ The answer-idea I wrote here is an easy and fun demo on the subject of ballistics. BTW, AR makes a good point about you not picking a Best, or even responding, to any of those chemistry project answers. Or do all these answers just suck? What the monkey? I mean, come on! We're dying out here! (figuratively speaking) Also some guidance on what kind of project idea you want might be helpful. Easy or hard? What branch of physics: mechanics, thermodynamics, E&M, etc?