This experiment demonstrates the effect of a water vortex, which looks a lot like a localized tornado in a bottle. The experiment consists of the child filling a plastic bottle up to the top, and then timing how long it takes for the water to empty out without the aid of any squeezing or shaking when the bottle is turned upside down. This is repeated a few times so an average time can be established, and then the experiment is performed again. This time, as the water flows out, the child swirls the water around in the bottle until a tornado-like, vortex effect appears. The water will come out of the bottle much faster because the vortex makes it easier for air to get in the bottle -- reducing the slow glugging which slows down the flow of liquid.
Another experiment creates a lava lamp effect in a plastic bottle with water, oil, food coloring and salt. The bottle is filled up two-thirds of the way with water and a few drops of food coloring are added. Oil is then poured in so it forms a layer on top of the water. Some salt is then poured into the bottle, which --- being heavier than water --- drags some oil down to the bottom of the bottle with it. Moments later, the salt dissolves and the oil rises back up to the surface, creating an effect similar to a lava lamp.
You can create a simplistic thermometer using rubbing alcohol, water, food coloring, a straw and some molding clay. The bottle is filled an eighth of the way up with water, then another eighth of the way up with rubbing alcohol, leaving a mixture filling around a quarter of the bottle. A few drops of food coloring are added, and stirred in. The straw is then left in the liquid, but not so it touches the bottom. The clay is used to stick the straw in place by covering the majority of the top of the bottle. The straw should poke out at the top, but be held by the clay. After this, if you get a child to put his hands on the bottle, he can see the colored liquid rise up through the straw.
You can compress airborne water molecules into a cloud using a 1-liter plastic bottle, some water and a foot pump with a rubber stopper attachment. The bottle is filled with a small amount of warm water, which is swirled around for a moment. The rubber stopper is put in the top and the foot pump is given five presses. Somebody needs to hold the stopper in place so it doesn't pop off. When the pump has been pressed five times, remove the stopper. A very faint cloud will appear. This effect is increased the more you pump the foot-pump, and even more if you use rubbing alcohol instead of water.