Is there an experiment that visibly shows the effects of metastability?
The standard demo for this is crystallization of sodium acetate trihydrate from its melt.  The trihydrate melts at 58 &deg;C.  The molten liquid is easily supercooled, after which point dumping in a few crystals of unmelted stuff results in a rapid solidification of the mixture.

Several videos on YouTube demonstrate the reaction; Nurdrage's is my personal favorite.