I got a valentine from a student on Wednesday that was a little foil package I was supposed to lay on a table and punch. That was supposed to break a packet inside that inflated a small balloon. It mostly worked (I had to help the balloon break free from the packaging).
It was very interesting. I had never seen such a thing before. The balloon made sizzling noises while it inflated, and it got cold. I was curious, so I cut the balloon open:
I did a little searching on the internet, and it appears to have been an acid/carbonate reaction that produces carbon dioxide (like vinegar and baking soda). Clever.