This is a very simple demonstration of surface tension. Place a coin on the benck top and add drops of water one at a time until no more can fit.
It can, however, be bade a little more sophisticated if you try using a variety of liquids of differing surface tensions, or look at the effect of detergents and other compounds on the surface tension.
Amazing Penny Experiment – Instructions
(There are no significant hazards associated with this experiment)