I want an activity using a syringe to demonstrate compressibility.
Okay, how about this one? Take a syringe, remove the needle part on the end. Put some plastic cement in the end where the needle used to go to seal it off (make sure you have the plunger part about 1/4 of the way in before you do this). Allow the cement to dry. Take a piece of wood, drill a hole roughly the size of the end of the syringe you cemented shut, and insert the syringe into the hole, where it is standing straight up. Now you can demonstrate the relationship between pressure and volume of a gas. Use one book on top of the plunger, the plunger goes down a little. Put another book on, and it goes down some more. Place a third book on top, the plunger is at its lowest point. This demonstrates Boyle's Law, which states the pressure and the volume of a gas are indirectly related. As the pressure becomes greater, the volume becomes less, and vice versa.