No clue what a science fair day is. I am guessing it is some sort of USA school thing?
If you are in primary school then doing one of those solubility experiments might be enough. Basically drinking bottles (eg coke, water) that you mix 2 substances in which do not dissolve. One bottle could be cooking oil and water, another water and sand and one for salt or sugar, or possibly even more. No matter how much you shake the oil and water, they will not mix permanently. No matter how much you shake the sand, even if it appears cloudy, it will eventually settle out. Although the salt and sugar will at first settle at the bottom, over time they will disappear and dissolve into the water (unless the water is saturated with stuff already). You could even mix the sand, water and oil into a single bottle and they should still separate!
The same experiment could work at highschool as well. However you will need to go into more detail about the science at work. For example the densities of water, oil and sand applying force to settle at different levels. With the sugar and salt dissolving you could explain the change in energy that occurs, and show that the resulting solution has less volume than its ingredients due to how the packing works. You could show how detergent acts as an emulsifying agent for oil in water so that it allows oil to dissolve in water. Obviously explain everything in detail, and throw out a few examples in real life (eg detergent used to help clean dishes).
If you want a physics one then you could do one of those diffraction grating experiments. Get white light and shine it through very fine uniform grating (might be hard to get?) onto a sheet of white paper. Due to how interference works, you will find that multiple spectrum appear on the paper. Changing the colour of light, eg with a filter will remove certain components from the spectrum.
You can also do one of those audible interference experiments. Basically two speakers far apart producing the same reasonably loud (not too loud, less than 90 dB for safety) low frequency sound wave. As you move your head around you should be able to sense it getting louder and quieter due to interference.
It might help to give us some context to what you are looking for. Eg what level of school you are at.