A water compass is a very simple device that indicates north, as any other compass, but is based on the direction a needle points while floating in water. Straighten a paper clip and slightly magnetize it by rubbing it against a magnet. Then poke it through a small piece of Styrofoam. This ensures that it floats in water. Place the compass in the water and it will point north. To test the compass, blow it so it points another direction and see if it reorients to point north again.
You can easily convert a mousetrap into a very strong catapult. Remove the trigger mechanism from the trap. You may need to use pliers. Tape a small plastic spoon to the arm. While the arm is at rest, the spoon should be face-down. Lift the arm so that you can tape the spoon securely. When taped, pull the arm back, load the spoon and release and your catapult will fire. Be careful; the arm is still strong.
To make a pinhole camera, you just need a Pringles can and tin foil. Cut about 2 inches from the bottom of the can. Poke a pinhole through the bottom. Tape the can back together but put the lid between the two tubes. Cover the pinhole camera to stop light from getting in. When you view things through the camera, you should find that the images are upside down.
Windmills are a very easy project for younger children. You just need two pieces of paper, a pin and a pencil. Cut two papers into two squares. Make diagonal lines and cut about two-thirds toward the center of the paper along these lines. Hold the two squares together and fold the corners toward the middle. Hold them in place with the pin, which is then pinned to the pencil. Blow against the paper and watch it spin.