Melt butter in skillet; add brown sugar and heat slowly, stirring until blended. Add crushed rice cereal. Toss well to coat with butter-sugar mixture. Remove from heat; add nuts and coconut.
Reserve ⅔ c. cereal mixture for topping. Spread remaining cereal mixture into an 8" square pan. Spread ice cream on top. Sprinkle the ⅔ c. cereal mixture over the ice cream.