I got this from a parenting message board, and my 2-year-old loved it.  Make a paste of corn starch and water. Put the paste on a cookie sheet, and in an area safe for messes, have your child get their hands dirty in the paste. When it starts to dry out, just add a little water.  We traced numbers and letters and shapes. For added fun, add a drop or two of food coloring.  It's so simple, but it's definitely entertaining for the toddler and preschool set.  And all of the paste washes out easily with water.