Everyone knows what happens when you combine water with sulfuric acid. In fact, you're warned to add the sulfuric acid to water and never to add water to sulfuric acid. But there are other chemicals that when mixed together cause a volatile reaction.
My first encounter with the properties of pure sodium in water actually occurred in a high school chemistry lab, when one of my classmates made the mistake of dropping sodium into a beaker of water. We never did find that beaker.
Frozen carbon dioxide, or dry ice, was also a fun item, specifically because it was colder than normal ice and it sublimed rather than pass into a liquid state. In the process of subliming, it would expand rather rapidly, with predictable consequences. It should also be noted that in real life, Ross could never have held a pellet of dry ice without his fingers suffering frostbite.
A final element that has interesting properties is magnesium, which was used to great effect in World War II. Burning magnesium was difficult to extinguish because it reacted violently to most typical fire-extinguishing agents. Throw water on it and it produced magnesium hydroxide and a whole lot of heat. It also burned hotter when either carbon dioxide or nitrogen were thrown on it. In short, magnesium fires had to be smothered with inert ingredients. Volatile reactions could also result from combinations such as picric acid and sodium hydroxide or ROTC students and SACC activists.