You'll need two balls of about the same size, but different weights, or mass; a ladder or step stool basically a height to drop the balls from ; and if you like a video camera. From walking coins on a vertical wire to levitating magnets, there are projects that seem centered more around magic than science.
This is the chemical name for it. Therefore when the air pressure changes, the dial turns. The high heat will evaporate the water allowing you to reuse the salt.
What I did was place a cardboard lid on the cups, and on top of the lids I placed insulation material for those two cups where insulation material was used — this way you have insulation all around. So you can buy it in stores during the winter months. The figure below shows the mechanism from an old barometer I took apart.
Inside the cups you place equal amounts of ice, by weight, from the same freezer. Galileo dropped a 10 pound ball and a 1 pound ball from the top of the leaning tower of Pisa, but you won't need to go to such great heights. Groups can compete to have the most effective parachutes, comparing the average falling times after completing multiple trials.
You can easily build one yourself, perhaps experimenting with only one fan instead of two.
The behaviour of a gyroscope is explained on my web page: Gyroscope Physics. Whatever set of materials you choose, it is important to limit the quantity to make the project a challenge.