Making an exploding lunch bag and blowing up a balloon with yeast are examples of some fun science experiment ideas. Both experiments are from the Science Bob website, which includes lists of needed items, directions and a scientific explanation.
To make an exploding lunch bag, put 1/4 cup of warm water into a zippered plastic bag. Add 1/2 cup of vinegar and 3 teaspoons of baking soda folded in a tissue. Drop the tissue into the partially zipped bag. Completely close the bag and stand back as the acid-base reaction creates a gas that causes the bag to pop.
To blow up a balloon with yeast, fill a 16-ounce plastic bottle with 1 inch of warm water. Add the contents of a whole yeast packet, and swirl them around. Add 1 teaspoon of sugar, and continue to swirl. Stretch out one balloon, and place it over the bottle neck. After 20 minutes, the yeast eating the sugar creates carbon dioxide, causing the balloon to fill.