Found this video on youtube showing a Cartesian Diver. I do NOT recommend showing students this video. Instead, make your own. I have had success by using 1-liter pop/soda bottles filling them with water and using a mostly full of water test tube inverted in the bottle. You will have to play around with how much air to leave in the test tube to get best results.
Oftentimes this demo is used to demonstrate some laws of science, but I use it to develop students thinking about density, particles, and air pressure. I ask them to make careful observations of the diver in order to try to figure out what causes the test tube to sink or float.
Student often try to take the easy way out and simply say, “When you squeeze the bottle it sinks.” I use additional questions, “Why would squeezing the bottle make it sink?” or “What changes about the test tube when you squeeze the bottle?”
Once students observe the little bubble inside the test tube getting smaller when the bottle is squeezed our discussion could go a number of ways depending on my goals with that class. I might ask, “Why does the bubble get smaller?” or “Why would having a smaller bubble cause the test tube to sink?” or “What happens to the density of the bubble when it gets squeezed?”
The hardest part about a demo like that is to not simply explain the phenomenon. You WILL have the students attention with this. Use this opportunity to get THEM thinking.