Aluminum Foil Ship Floating on Hexafluorid (Video)
Here is an interesting clip showing an aluminum foil ship floating above ground on hexafluorid, a non-toxic gas that is 5.11 times as dense as air. Hexafluorid is usually considered as pretty innoffensive, but be careful if you fool around with its byproducts, they can be extremely dangerous. Enjoy the show!
Another interesting point is that inhaling this gas will make your voice sound deeper. Sound does not travel as fast in this medium. Helium can transmit sound very rapidly, hence making your voice squeaky (Source: Neatorama)