This experiment is an oversimplification, I know, but it gets idea across for younger students.
Here I react three metals -- aluminum, zinc and copper -- with the commonly available muriatic acid (hydrochloric acid). It's a cheap and easy way to visualize the reactivity of different metals, using easy to find materials. Plus, it's fun. To extend this, you can try different metals, for example an iron nail. See what you have in your home or lab. Then look at an activity series chart and the Periodic Table and draw conclusions.
Safety notes: This reaction produces noxious gas and heat. Goggles and gloves should be worn. Seriously.
Sorry for the video quality. First time with a new camera. Plus I have a sinus infection, so my voice sounds terrible.