Line 2 large baking sheets with parchment paper or silicone baking mats. Set a very large mixing bowl on the counter so you have plenty of room to stir the snack mix.
Add the plain popcorn, caramel popcorn, cinnamon cereal, mini pretzels, dried apple chips, and roasted salted nuts to the large mixing bowl. Gently toss with clean hands or a big spatula to distribute everything evenly, breaking any very large apple chips into popcorn-sized pieces.
6 cups plain or lightly salted popcorn, 3 cups caramel popcorn, 2 cups cinnamon cereal, 2 cups mini pretzels or pretzel twists, 1 1/2 cups dried apple chips, 1 cup roasted salted nuts
In a medium saucepan over medium heat, melt the unsalted butter. Stir in the brown sugar, light corn syrup, salt, ground cinnamon, and ground nutmeg (if using). Cook, stirring, until the sugar dissolves and the mixture looks glossy and well combined.
1/2 cup unsalted butter, 3/4 cup packed brown sugar, 1/4 cup light corn syrup, 1/2 teaspoon fine sea salt or table salt, 1 teaspoon ground cinnamon, 1/4 teaspoon ground nutmeg
Bring the caramel mixture to a gentle boil over medium heat. Let it bubble for 3–4 minutes, stirring occasionally, until slightly thickened and it clings lightly to the spoon and smells like deep caramel.
Remove the saucepan from the heat. Stir in the vanilla extract and baking soda. The caramel will foam and lighten in color; this is normal and helps create a light, crisp coating.
1 teaspoon pure vanilla extract, 1/4 teaspoon baking soda
Quickly pour the warm caramel evenly over the popcorn mixture in the large bowl. Working carefully and quickly (the caramel is hot), use a large spatula to gently toss and fold the mix until everything has some caramel on it. It does not need to be perfectly evenly coated.
Spread the coated snack mix onto the prepared baking sheets in an even layer. While the caramel is still tacky, sprinkle on the caramel bits, white chocolate chips or cinnamon chips, and fall sprinkles if using, so they adhere to the mix.
1 cup caramel bits or chopped soft caramels, 1 cup white chocolate chips or cinnamon chips, fall sprinkles
Let the caramel apple snack mix cool completely at room temperature on the baking sheets, about 30–40 minutes, until the caramel is set and the mix is crisp to the touch.
Once fully cool, break the snack mix into clusters. Transfer to a serving bowl for immediate snacking, or pack into airtight containers, jars, or treat bags for storage or gifting.