Hi, Don't have a lot of ideas for a specific project, but I suggest that whatever you decide on, do some thinking about planning before jumping. Not TOO much..But:
I have found that various TIME things can be interesting. Since Arduino can handle time well, you can do lots of strobe light things. users spin tops and fidget spinners, and change the strobe frequency to stop motion. A stream of water droplets appears to be stopped. 2 watt white LED modules work well.
Then a challenge: Drop different lengths of wood (like 2x4x??) down a large PCV pipe so they drop out the bottom. Tell how LONG they are with an arduino.
Or the classic reaction time test...
Or see THIS about things high school kids did with kits I made for their school system...
This planning page is really helpful. I cant count how many times it felt like tearing your hair because everything is okay but nothing works.. Thanks.
I thought human eyes perceive motion differently than a camera, why would a specific frequency make it look still? Or do i have to put the water in a black box with window?
Again, people judging the contest are going to ask "so how is this useful?"