Prepare the base. In a saucepan, gently heat the corn syrup and peanut butter over medium heat, stirring until smooth and fully combined.
Mix the dry ingredients. In a large mixing bowl, combine the crispy rice cereal and oats. Pour the warm peanut butter mixture over the dry ingredients and stir until everything is evenly coated.
Shape the mixture. Line an 8x8-inch baking dish with parchment paper, leaving some overhang for easy removal later. Press the cereal mixture into the dish firmly and evenly. A piece of parchment on top and the flat bottom of a measuring cup can help achieve a smooth surface.
Add the chocolate layer. Melt the almond bark according to package instructions. Pour over the cereal base and spread evenly with an offset spatula. Refrigerate for about 30 minutes, or until the chocolate has fully set.
Cut into trees. Once firm, lift the mixture from the pan using the parchment overhang. Cut the square in half, then slice each half into 5–6 triangles. These will be your trees.
Decorate. Using the green icing, pipe zigzag lines across each triangle to resemble the branches of a tree. If desired, add sprinkles or silver dragees for ornaments, or even a small candy star at the top.