In the cartesian diver, you fill a 2 Liter soda bottle to the brim with water and place a glass dropper pipette half filled with water inside then screw the top back on.  If you place the correct amount of water in the pipette the dropper should float, but when you gently squeeze the side of the two liter bottle, the dropper will sink to the bottom.  When you release, it will rise again.  As for physics concepts, you can talk about buoyancy, density of materials, contrasting solids, liquids, & gases, and pressure in hydraulic systems.  To further the instruction, you can initially introduce this as the "first voice-command device in the world" from Ancient Greece.  You can then guide them through examination and questioning about the device as you lead into the physics concepts, instilling a sense of intrigue and discovery.