Grease an 8×8-inch square baking dish (glass or metal) and set aside.
In a large pot over medium-low heat, melt the butter and unwrapped caramels, stirring until smooth.
Add the marshmallows to the pot and stir continuously until completely melted and smooth; remove the pot from the heat.
Stir in the vanilla and salt until combined.
Add the multigrain Cheerios, chopped peanuts, chopped banana chips, chopped pretzel sticks, and M&M’s to the pot and mix with a sturdy spoon until everything is evenly coated and sticky.
Transfer the mixture to the prepared baking dish; butter your fingers or use a piece of parchment and press the mixture firmly and evenly into the dish.
Let the bars cool to room temperature, then cut into squares and serve.