A balloon is dropped in boiling water, and another balloon is cooled with liquid nitrogen to demonstrate the effect of temperature on volume of gas in a balloon.
A 4 liter beaker on a hot plate, with a liter of water boiling within. A prepared balloon, filled with air and tied so that it just fits in the beaker, is dropped into the beaker. The balloon expands to seal the beaker and is forced up by the vapor pressure, ending up on top of the beaker, too large to fit inside.
A foam ice bucket will hold a helium ballon that is large enough to seal around the inside of the bucket. By burping out air from beneath the balloon as it is settled into the bucket, the air pressure difference will hold the helium balloon down.
Liquid nitrogen is poured slowly from a transfer Dewar over the top of the balloon, making sure to cover the whole thing, pouring about a liter in total. When the helium balloon is shrunken, and floating in liquid nitrogen, it can be extracted carefully and placed on the lab bench. The balloon will quickly re-inflate, and float away to the ceiling. Huzzah.