Melt margarine in large saucepan over low heat. Add marshmallows and stir until completely melted. Remove from heat.
Add Kellogg's(R) Rice Krispies(R) cereal. Stir until well coated.
Using buttered spatula or waxed paper, press mixture evenly in 15 1/2 x 10 1/2 x 1-inch pan coated with cooking spray. Cool slightly. Using 4-inch tree cookie cutter, cut into tree shapes. Decorate as desired with frosting and red cinnamon candies. Best if served the same day.