You can build a four-wheeled ATV!
You can design the wheels and build the propeller.
Build your own teeter-totter and experiment with balance or as a scale.
By adding pulleys to the top of the bucket the mechanical advantage is increased, greatly reducing the amount of force needed...
With the addition of a spool and crank handle, you can create a way for your pulley system to work for you.
Cranes come in all shapes and sizes. In this video see how you can build your own crane with a fulcrum to lift heavier objects.