PREPARE cake batter and bake in 13x9-inch baking pan as directed on package. Cool completely.
INVERT cake onto large platter; remove pan. Using a serrated knife, cut and scoop out a shallow rectangle from centre of cake, leaving a 2-inch border of cake on all sides and a thin layer of cake on the bottom. Reserve removed cake for snacking or other use.
STIR boiling water into dry jelly powder in large bowl at least 2 min. until completely dissolved. Add ice cubes; stir until jelly begins to thicken. Remove any unmelted ice. Pour thickened jelly into centre of cake. Refrigerate 4 hours.
FROST borders of cake with whipped topping. Decorate with remaining ingredients as desired to resemble a swimming pool. Store in refrigerator.