Preheat oven to 350º F. Grease and flour two 9"(or 8") round cake pans. Place 2 cups of Fruity Pebbles Cereal in small bowl. Set aside. Prepare cake mix as directed on package and fold in 2 cups of cereal. Divide batter equally between 2 cake pans. Bake 25-35 minutes for 9" pans (30-40 minutes for 8" pans) or until the top springs back when touched lightly and a toothpick inserted in the middle comes out clean.
Allow cake to cool 10 minutes in pans. Invert cakes on cooling rack and cool completely. Once cooled the cupcake is ready to assemble.
Place one cake layer on plate and frost top evenly with ½ can of frosting. Sprinkle with 1 cup of Fruity Pebbles Cereal. Top with second cake layer. Frost sides of cupcake with ½ can of frosting-heavier at the top edge to create a slightly flared look. Adhere pretzel sticks, chocolate sticks (or other cookies/candies) around sides of cake, close to each other. Trim as necessary to resemble a cupcake liner.
Tint remaining frosting with desired coloring and spread on top to resemble giant cupcake. Sprinkle top with remaining 1 cup of cereal right before serving.