Use foam tubing or old toilet paper rolls to make a roller coaster for marbles or small cars. Cut the tube in half lengthwise and attach your pieces together with some heavy tape or glue. Create a large ramp on one end for the coaster to begin at and even experiment with building a loop. Roller coasters use gravity, kinetic energy, and friction to work. Try to figure out how much kinetic energy is necessary to make the loop successful.
Use two large plastic soda bottles to create your own vortex by filling one of the bottles with water and taping the two bottles together by connecting them at the narrow openings at the top. After making a very tight seal, turn the taped bottles over and watch the whirlpool that forms as the water drains from the top bottle into the second. The fluid on the outside has to move faster than the fluid on the inside to keep up, creating the vortex. This is the same principal at work in a tornado.
You can test how heat can affect aluminum. Take an empty soda can and fill it with 1/4 an inch of water and place it on a hot plate or stove. Bring the water in the can to a boil, and place a large bowl with several inches of cold water next to the can. Make sure there is a little water left in the can and use some tongs to lift the can, turn it upside down, and submerge it into the water. The can should be instantly crushed as the steam inside the can changes back into liquid water, creating a vacuum as the can becomes submerged.
Experiment with how sound vibrations travel by building a simple string telephone out of two small cans and some string. Tie a toothpick to the end of a long piece of string, and insert it through a very small hole in the bottom of a coffee can. The other end of the string should be fashioned similarly to the inside of another coffee can. By pulling the string tight and speaking into the coffee can, sound waves can travel through the string and be heard by placing your ear inside the opposite can. The sound can move faster through the string than through the air.