Spray very large microwavable bowl with cooking spray. Mix cereal, popcorn and pretzel crackers in bowl.
In medium microwavable bowl, microwave marshmallow creme and butter uncovered on High about 1 minute, stirring after 30 seconds, until creme and butter are melted and can be stirred smooth.
Stir, pour over cereal mixture, stirring until evenly coated. Microwave uncovered on High 2 to 3 minutes, stirring every minute, until mixture is glazed. Cool 5 minutes; stir in candies. Spread on waxed paper to cool. Break into chunks. Store covered.