There is an extremely entertaining experiment that can be done with two simple ingredients found in the local supermarket: Mentos mints and Diet Coke (it is more effective than original Coke). The experiment procedure is as following:
After opening the bottle, quickly drop the Mentos in to the Diet Coke. Run.
As soon as the Mentos falls in, the Coke spurts foam explosively, which shoots up to great heights. Depending on the temperature of the Coke (the warmer the better) and the number of Mentos mints (it is more effective to thread them and drop them all at once), the pillar of foam can rise to a few metres.
This fascinating phenomenon is not caused purely by a chemical reaction, but has more to do with physics. Mentos mints are coated with menthol, which has numerous microscopic dents. When it enters the Coke, the dissolved carbon dioxide forms bubbles that collect in these pits. As they collect, the bubbles expand until the pressure builds past a certain level, causing an explosion.
The most important point is that if this experiment is not performed outside, one could end up cleaning a sticky room for days.