Please, be very original!
Tried websearching? Tried asking your teacher for some general ideas?
A rube goldberg machine! Be as creative as you can with the energy transformations.
You do the originality in this, it's your project. Some ideas:www.instructables.com/Science Fair/ L
+1 If I come up with a very original idea, I'll submit it under my own name. If somebody else comes up with an original idea and needs help implementing it, that's a whole different thing.
Cat in a box? Not an original idea, but has anybody actually done it? : D
just an idea but depending on your age you can look up some low cost physics experiments and do that