Coat 17 x 11 x 2-inch baking pan with cooking spray. Combine cereal, popcorn, pretzels and peanuts (if desired). Set aside.
In heavy 3-quart saucepan combine brown sugar, butter and corn syrup. Bring to boiling over medium heat, stirring constantly. Reduce heat. Simmer for 5 minutes, stirring constantly. Remove from heat. Stir in vanilla and baking soda. Immediately pour over cereal mixture. Stir until evenly coated.
Bake at 250°F about 1 hour or until deep golden brown, stirring every 15 minutes. Spread on wax paper. Separate clumps. Cool completely. Store in airtight container.