PREPARE cake batter and bake in 13x9-inch pan as directed on package. Cool in pan 15 min. Invert cake onto wire rack; remove pan. Cool completely. Transfer cake to cutting board; cut into 9-inch square. Place on large platter or cake board. Reserve cake scraps for snacking or other use.
TINT Cool Whip with food colouring to desired shade of green; spread onto top and sides of cake square.
SPRINKLE wafer crumbs in diamond shape in centre of cake to resemble a baseball infield; draw base lines with white icing. Add a marshmallow piece for the home plate and each base. Decorate cookies with coloured icing to resemble baseball players; arrange on cake. Refrigerate up to 2 hours before serving.