Prepare and bake cake mix 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 center 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 gelatin mix in large bowl at least 2 min. until completely dissolved. Stir in cold water. Refrigerate 1-1/4 hours or until slightly thickened (consistency of unbeaten egg whites). (Or, see Tip below for quicker set method.)
Pour thickened gelatin into center of cake. Refrigerate 3 hours or until set.
Frost borders of cake with whipped topping. Decorate with remaining ingredients as desired to resemble a swimming pool. Store leftover cake in refrigerator.
