PREPARE cake batter and bake in 13×9-inch pan as directed on package. Cool completely. Invert cake onto platter; remove pan. Carefully hollow out center of cake, leaving thin layer on bottom and about 2-inch-wide irregular-shaped border on sides. Crumble removed cake. Reserve 2 Tbsp.; mix remaining crumbled cake with chopped cookies.
SPREAD about 1/2 cup COOL WHIP onto bottom of hole in cake. Add boiling water to gelatin mixes in large bowl; stir 2 min. until completely dissolved. Add ice cubes; stir until gelatin starts to thicken. Remove any unmelted ice. Stir in reserved cake crumbs; spoon over COOL WHIP in cake. Add SWEDISH FISH candies as shown in photo. Refrigerate 3 hours or until gelatin is firm.
FROST sides and border of cake with remaining COOL WHIP; press enough cookie crumb mixture into COOL WHIP to evenly cover COOL WHIP. Sprinkle remaining crumb mixture onto platter around cake.
ROLL marshmallow into 3-inch-long piece; fold lengthwise in half and flatten slightly to resemble ghost. Place on cake; use decorating gel to make eyes. Decorate cake with remaining ingredients to resemble photo.