In a saucepan, melt margarine over low heat. Add marshmallows and stir until completely melted. Cook over low heat 3 minutes longer, stirring constantly. Remove from heat. Add crispy rice cereal. Stir until well coated. Press mixture into a 13 x 9" pan, coated with cooking spray. Remove from pan and place on foil-lined cutting board. Using red decorating icing, make straight line 9" across the center of mixture. On left side of line, make heart-shaped design using frosting. On the right side of line , write your own personalized message using red decorating icing. For a finishing touch, trim edges of greeting using frosting.