For a car to move forward, some source of energy or push is needed: an engine, a downward slope, even a balloon. Instruct your students to construct simple cars made of cardboard or Styrofoam, with a straw inserted into a balloon as a source of power. Blowing up and releasing the balloon will cause the car to move, demonstrating Newton's third law of motion. For more advanced students, ask them to calculate the "miles per gallon" of each car by measuring how far their car travels on a single balloon full of air.
Newton's Cradle or Newton's Swing is a demonstration of Newton's third law, often used as a desktop distraction. To make a Newton's Cradle, hang evenly spaced balls from a central rod. When a ball on one end is pulled back and released, it will strike the next ball, which will strike the next, and so on. This demonstrates how energy is transferred as predicted by Newton's third law. For more advanced students, replace one of the balls with something heavier or lighter. For example, if the cradle was constructed with tennis balls, how would adding a (solid, heavier) baseball change things? Ask students to predict what will happen, then carry out an experiment to verify or disprove their predictions.
Place your students in roller skates or roller blades (lacking that, place them on chairs with wheels). Ask them what will happen if they throw a ball while on wheels. The action of throwing a ball causes a reaction of moving backward on wheels. Ask students to predict what will happen if they throw heavier or lighter balls, and have them test their predictions.
Water coming from a spout can also demonstrate Newton's third law of motion. Poke five holes in a half-gallon paper milk carton, one at the top and one in the bottom left corner of each of the four faces of the carton. Tie a string through the top hole and, while covering the bottom holes, fill the carton with water. Uncover the holes and the carton should spin as the water leaves the container. Ask students why this happens and whether placing the holes in different locations will change how the carton rotates.