Keep it simple. I think your examples might be too advanced. Failing is boring.

You know these 'sticks' that you hold an ice cream with?

Bend those into a propeller. Wet them with water if they crack. Drill a hole (or cut with knife) through the middle and put it on a small electric motor and attach a battery or power source. Spin it up and watch the propeller fly off.

Just give him or her the things and say the goal. And that you don't know if it will work. The hard part is figuring out to bend with water and to get the right fit on the hole to the axis.