Coat the inside of a 9-inch x 13-inch baking dish with non-stick cooking spray. Set aside.
In a large microwave-safe bowl, combine the marshmallows and butter. Place in the microwave and heat for 2 minutes, and then stir. Heat in 30-second intervals as needed until smooth.
Pour the Cheerios into the bowl and stir until coated. Pour the Cheerio mixture into the prepared baking dish.
Grease your hands and use them to press the cereal into the pan in an even layer. (Don't press too hard though. You don't want to make the bars super dense.) Sprinkle the chocolate chips and candies over the Cheerios.
Pour the white chocolate wafers into a microwave-safe bowl. Heat in 30-second intervals, stirring between each heating, until melted and smooth. Drizzle the melted white chocolate over the bars.
Refrigerate for 1 hour until set. Then slice into bars and serve. Store any leftovers in the fridge for up to 7 days or freeze the cereal bars for up to 3 months.
Notes
M&Ms were used for the "candy" for nutrition calculation purposes.
If you're using larger candy, like Snickers or Reese's Peanut Butter Cups, I recommend chopping them into smaller pieces before topping the bars.