Use a glass of white vinegar and a glass of water to illustrate Cain and Abel's hearts. Add green food coloring to the vinegar (this is Cain). Put a spoon of baking soda in both cups- the vinegar will bubble and boil over.
There are some great activities for helping children to interact with Bible stories on this site. Includes: Abraham, Tower of Babel, Creation, Adam & Eve, Cain & Abel, Noah's Ark, Jacob & Esau, Moses, Jonah and more. April's Homemaking: Simply Sunday School