Preheat oven to 350F. Grease a 9x9 baking dish with cooking spray.
In a large bowl, combine the brownie mix, egg, oil and water, and stir until well-blended. Gently stir in caramel popcorn, pretzels and peanuts. Spread into greased pan.
Bake for 30-35 minutes. Brownies are done when a toothpick inserted in the center comes out clean. Let cool on a wire rack completely before cutting into squares.