You can demonstrate how vibration is used to spin an object and make a working helicopter propeller at the same time. You will need the bottom rod of a coat hanger, pliers, an empty cereal box, a pencil, and some tape. You can break the coat hanger rod off by bending it with pliers on each end, back and forth, until it breaks. When you have one length of wire, use the pliers to create little bumps about 10 mm in height all along it. Cut a piece of cardboard from your box that is 2 inches wide and 4 inches long. This will be your propeller. Make a hole in the center of the propeller and skewer it onto your wire rod. Use the tape to secure the end of the rod so the propeller doesn't fly off. Holding the rod at the other end, scrape the pencil along the length of the rod over the bumps. The vibration from the pencil over the bumps will cause the propeller to spin. Think of it like the propeller is a tiny hula hoop and the rod is the person. The vibration causes the rod to spin, making the propeller spin.
Another simple homemade science experiment demonstrates air pressure. Because we can't see air, we take for granted the space it takes up. You can illustrate this space with a balloon, a straw, some duct tape and a glass bottle. First, cut the rolled end off the balloon. Place the balloon over the straw and tape it closed. Demonstrate that you can blow into the straw and easily blow the balloon up. Now, place the balloon with the straw still attached into the bottle. Seal the mouth of the bottle around the straw with more tape. Again blow into the straw to try to blow the balloon up. This time the balloon will not blow up no matter how hard you blow. This is because the air pressure inside the bottle will not allow the balloon to expand without letting air out of the mouth of the bottle.