When entering a Science Fair to make an impression, my rule has always been "GO SIMPLE!" and impress people with the science already around them.
I learned this when I won a 6th-grade science fair with a topic titled "The Chemistry and Physics of Toast."
I did a detailed analysis and description of everything involved in what it takes to turn a slice of bread into golden brown toast.
Sounds simple, but when it's all said and done, the toaster's workings alone are quite impressive, but the chemistry of the browning process in the bread is over the top!
Look around you and find something simple like that and ask YOURSELF if you understand "how it works." If not, it'll make a great topic for a Science Fair!
GL!