You can use 2-liter bottles to create simple instruments for preschoolers to play. Begin by letting them bang gently on empty bottles with a thick crayon or marker to make a noise. After all the kids have gotten a chance to try that, fill bottles with different levels of water and let them take turns banging on different bottles as you hold the bottle necks to keep them steady. Talk about the different sounds that the bottles make when there are different amounts of air left inside them.
Fill one bottle two-thirds full with water. If desired, add food coloring or glitter to the water. Place a piece of duct tape over the mouth of the bottle and puncture a 1/4-inch hole in the center with a sharp pencil or scissors. Carefully attach a second bottle to the first, neck to neck, using duct tape and making sure to form a strong and leak-proof seal. Turn the bottles over so that the water is in the top half, swirl them briefly and watch the water form a vortex as it drains into the empty bottle.
This experiment uses the same chemical reaction as a science fair volcano, but harnesses it in a different way. Pour about 1/2 cup of vinegar into the bottom of a bottle. Wrap 1 teaspoon of baking soda in a small piece of tissue, and then drop it into the vinegar and quickly stretch the neck of a small balloon over the mouth of the bottle. The carbon dioxide produced in the chemical reaction should inflate the balloon. You may need to wet the balloon's mouth to ensure a tight seal.
Transforming a 2-liter bottle into a terrarium will allow children to observe a working microcosm of the water cycle. Cut the bottom 4 inches off a 2-liter bottle. Place an inch of pebbles in the bottom, and then cover it with 2 inches of soil. If desired, you can lay thin layers of activated charcoal and sphagnum moss between the pebbles and the soil. Plant a few flower seeds in the soil, add enough water to moisten the soil and slide the top half of the bottle down over the bottom, leaving the cap tightly closed. Place in the sun.