i saw this dispenser for cereal where you turned a handle, and it rotated a screw. The screw would move the cereal forward in it's threads and drop it out of an opening (does this make any sense??)maybe use something like this, except it could be powered by a water wheel or something like that
If you have a bunch of screws together on a slightly inclined plane, you can have something hit the head of the first screw. That screw would circle around down the plane and hit the second screw, and it would be like a domino effect. The last screw could then do something cool, like hit two metal stubs to activate a battery which powers a fan to continue.
I wish I had fun assignments like this in my school!