Place frozen banana pieces, frozen berries, peanut butter, and 1/2 cup water in a high-speed blender. Pulse until smooth, adding more water (one tablespoon at a time) if needed to keep the blender going. You may need to stop the blender and scrap down the sides/remix the ingredients as needed. Once mixture is completely smooth, divide between bowls and top with desired toppings like kiwi slices, banana slices, toasted coconut flakes, and berries then ENJOY!