A little physics demo...
Use a variety of vapor refrigerant to inflate a few balloons. Use the appropriate color balloon for each refrigerant, if you can. Tie each balloon.
Place each of them in a small ice chest with a block of dry ice, ahead of time. A pound or two is enough. The balloon must be in contact with the dry ice. The balloons will shrink to nothing as the vapor refrigerant condenses into liquid. You can see it through the balloon. Quite a bit of liquid collects in the bottom of each balloon.
Remove them, one at a time, and toss one to each of your students.
They will re-inflate as the refrigerant warms & boils.
You can put them back into the ice chest, and let them shrink again.
Work this into a discussion about gas laws, conservation of energy, state of matter, superheat, subcooling, saturation, whatever...
Try it with a balloon inflated with air, nitrogen, CO2, whatever else you have. Some will shrink, some won't.
You can tell a noticable difference in the weight of the balloon, if compared with one inflated with air.
Try throwing them. R-134a is really heavy.
Official disclaimer: The refrigerant will absorb heat at a great rate. Don't let them frostbite themselves.
When you get finished with the demo, recover the refrigerant back into your DOT approved cylinder.
You figure out how. I did.
An interesting variation is R-290.
You can smell it through the balloon, and it will leak down in a short time. They will want to know why.
Here's an interesting link about R-290:
In addition, you can use the dry ice for other demos. I am amazed at how many of my students have never seen dry ice.
I saw this done with liquid nitrogen at NASA, but I don't know of anybody else that does this with dry ice.
Try it, and let me know how it works for you.