Preheat oven to 300°F. Line a rimmed baking sheet with parchment paper. Stir together corn cereal, pretzels, cheese crackers, rice cereal, and wheat cereal in a large bowl. Whisk together butter, brown sugar, honey, and salt in a small bowl until smooth. Pour butter mixture over cereal mixture; stir to thoroughly coat. Spread mixture in an even layer on prepared baking sheet.

Bake in preheated oven until mixture is light golden brown and toasted, about 30 minutes, stirring every 10 minutes. Remove from oven. Let cool completely, about 30 minutes, stirring occasionally and breaking up clumps with your hands. Stir in marshmallows and candy-coated peanut butter pieces. Store in an airtight container at room temperature up to 1 week.