We have been doing this for a while now, but I had the chance to snap a picture the other day.
Materials: ice, spoons, bowls, water
Throw some ice cubes into a bowl of water. We have a couple fun ice cube trays from IKEA (stars and fish) but we throw the regular old cubes in there too (they last longer). Give them each spoons and tell them to stir until the ice is all gone.
A couple of times the water has gotten too cold and the ice stops melting -- add a cup of warm/hot water if this happens. Or you could leave it be and just let them wait it out.
Variations: you could do colored ice cubes. you could pile up some snow and have them use a spoon to pour hot water and make the snow melt. You could freeze objects in the ice and stir until they are free! I have seen somewhere a large bowl filled with ice with lots of little trinkets suspended in it --- like fossils. USe warm water to melt the ice away from the objects...