Student on cart with Fire Extinguisher
Place yourself or a student (you'll get many volunteers) on the floor cart with the gas cylinder placed between their legs. Point the nozzle away from the student, toward the front. Push the cart/student backwards, then have them "crack" the valve. The gas escapes and will slow them down to a stop, then push them back toward you with increasing velocity. Then have them close the valve after they start backward. Try not to leave the valve on any longer than necessary because the tank will be empty after about 30 seconds. If enough is left, you can repeat the demo with someone else or add a second small student (if room) to demonstrate Newton's 2nd law as well.