Give kids a basic cookie recipe - I often use the chocolate brownie recipe on the back of a bag of chocolate chips.\
Ask studets to substitute ONE ingregient in the original recipe - e.g. marshmallows for chocolate chips, whole wheat flour for white flour and bring the recipe to school. Have a tasting party and evaluation sheets. Have students make a chart of the recipes they enjoyed best.
brainstorm other substitutions that might be made to the original recipe. Remember to change only one ingredient so that the variable are controlled.