Everyone knows the effect of helium gas. Who hasn't inhaled a party balloon and had fun talking like Donald Duck? Kids today are at least familiar with this phenomenon.
At Jay Leno a couple of months ago, Josh Duhamel demonstrated the behaviour of a different gas heavier than the components of air. He described it as "anti-helium", the correct name is SF 6 (Sulfur Hexafluoride). This is a nice demonstration at science shows. The gas is harmless to people (in moderate doses), but harms the ozone layer. So please consider the risks for the environment if you wish to use this demo.