Build a device that will carry a seed at least 3 feet from where it is launched.
Design and build a powerful bird wing that can spin you around in an office chair when you flap it.
Build a device that will slow down your payload as it falls so it lands gently when dropped from 8 feet high.
Design a stomp rocket propelled by air pressure that can fly at least 10 ft in the air.
Use a simple chemical reaction to launch a small object.