Special equipment: an 11-by-15-inch baking pan, a 13-by-19-inch cardboard cake board or cutting board, a 12-by-17-inch baking sheet, a pastry bag with a small round tip and 6-inch bamboo skewers
For the cake field and stadium flags: Make and bake the cake in an 11-by-15-inch baking pan according to the package directions. Let cool completely.
Meanwhile, line a baking sheet with parchment paper. Cut the colored taffy into triangles and wrap the wide end of each around a skewer. Place on the prepared baking sheet and freeze for 2 hours.
Remove the cake from the pan and trim the top so it's flat. Place the cake cut-side down on a 13-by-19-inch cardboard cake board or cutting board. Frost the cake with the vanilla frosting. Trace a rectangle approximately 7 inches by 11 inches in the frosting in the middle of the cake. This will be the field that gets covered in green coconut grass.
Add the coconut to the bowl of a stand mixer fitted with the paddle attachment. Turn the mixer on low, add the food coloring and mix until the coconut is completely green. Press the coconut into the rectangle to make the field, leaving a 1 1/2- to 2-inch border around the cake for the bleacher seats.
To make the yard lines, cut thin strips of taffy and roll into thin ropes about 7 inches long; make 9 of these. Place 1 in the center of the field, then evenly space 4 on either side of the center line.
For the rice cereal treat stadium walls: Spray a 12-by-17-inch baking sheet with nonstick cooking spray.
Melt the butter in a large saucepan over medium heat. Add the marshmallows and cook, stirring, until completely melted. Remove from the heat, add the cereal and stir to combine. Press the mixture into the prepared baking sheet. Let cool.
Cut the rice cereal treats into three 3 1/2-inch-wide strips. Cut 1 strip to a length of 9 inches and leave the other 2 strips 17 inches long. Wrap the strips around the cake, pressing them into the side of the cake to help them adhere.
For the bleacher seats and fans: Put the frosting in a piping bag with a small round tip. Place 3 whole wafer cookies end to end along each sideline (the area without the coconut) and butted up against the cereal treat walls. Place 2 whole wafer cookies end to end in each "end zone" of the cake. Place a few dots of frosting on each cookie and stack another whole wafer cookie on top of each. Place small dots of frosting on the back edge of the second cookie layer and top each with a cookie half. Place the remaining 10 cookie halves in front of the stacked cookies and butted up against the coconut field. Place the candy dots on the bleacher seats to make the crowd/fans.
For the goal posts: Line a baking sheet with parchment paper. Cut four 2-inch pretzel pieces for the uprights. Cut two 1 1/2-inch pretzel pieces for the crossbars. Cut two 3- to 4-inch pretzel pieces for the bases.
Melt the chocolate melting wafers in a double boiler. Coat the 2 crossbar pieces in the candy coating and place on the prepared baking sheet. Coat the 4 upright pieces in the candy coating and place the uprights at either end of the crossbar pieces. Coat the base pieces in the candy coating and place one in the middle of the each crossbar. Refrigerate until the coating hardens. When set, carefully place a goal post in each end zone of the cake.
For the players and football: Place the gummy bears on the field as players. Cut out little lines from the taffy and press them on the chocolate-covered almond to make laces. Place the football in the center of the field. Position the stadium flags in the cereal treat walls.