Set out a large mixing bowl — at least 5-quart capacity. Measure all ingredients into small individual bowls before you begin. This is called mise en place, and it makes assembly fast and stress-free. Checkpoint: You are ready to move on when every ingredient is portioned and your large bowl is empty and waiting.
Add 2 cups of Chex cereal, 1½ cups of mini pretzels, and 1 cup of salted peanuts to the large bowl. Using a rubber spatula, toss gently for about 1 minute until these sturdy ingredients are evenly mixed with no clumps. Checkpoint: You are ready to move on when the pretzels, cereal, and nuts are evenly distributed throughout the bowl.
Add 1 cup of popcorn and ½ cup of roasted pumpkin seeds. Using a scooping motion from the bottom of the bowl upward, fold these in gently for about 1 minute. Do not stir — this will crush the popcorn. The popcorn pieces should remain intact and visible. Checkpoint: You are ready to move on when popcorn is distributed without visible breakage.
Add ½ cup of dried cranberries and ½ cup of mini marshmallows. Fold in carefully for about 1 minute. You should see pops of red and white evenly spread through the mix. Checkpoint: You are ready to move on when both ingredients are visible throughout the bowl without clumping together.
Add ½ cup of Halloween M&Ms (or Reese's Pieces), ⅓ cup of candy corn, and ¼ cup of chocolate chips. Fold in gently just 3 to 4 times — no more. Over-mixing at this stage breaks the candy and muddies the colors. The mix should look festive with orange, yellow, brown, and white colors clearly visible. Checkpoint: You are ready to move on when candy pieces are distributed but the colors are still distinct and bright.
Do a quick taste test. If the mix feels too sweet, fold in an extra small handful of pretzels or nuts. If it needs more color and candy flavor, add a few more M&Ms. Scatter candy eyeballs on top now if using, for decoration only — do not fold them in. Checkpoint: You are ready to serve or store when the mix tastes balanced and looks colorful and festive.
Serve immediately in the large bowl, or transfer to a zip-lock bag or airtight container. If you plan to store the mix for more than a few hours, leave out the marshmallows and add them right before serving to prevent them from drying out and becoming hard.