Preheat the oven to 275°F (135°C). Line a large rimmed baking sheet with parchment paper or a silicone baking mat to prevent sticking and make cleanup easier.
In a very large mixing bowl, combine the rice Chex, corn Chex, wheat Chex, mini pretzels, pecan halves, and pumpkin seeds. Gently toss until evenly mixed so there is a good variety in every scoop.
3 cups rice Chex cereal, 3 cups corn Chex cereal, 2 cups wheat Chex cereal, 1 1/2 cups mini pretzels, 1 cup pecan halves, 1/2 cup pumpkin seeds
In a medium microwave-safe bowl or small saucepan, melt the butter. Whisk in the brown sugar, maple syrup, pumpkin pie spice, cinnamon, vanilla, and salt. Stir until the sugar begins to dissolve and the mixture looks smooth and glossy.
1/2 cup unsalted butter, 1/2 cup light brown sugar, 1/4 cup pure maple syrup, 2 teaspoons pumpkin pie spice, 1 teaspoon ground cinnamon, 2 teaspoons pure vanilla extract, 1/2 teaspoon fine sea salt
Slowly pour the warm pumpkin spice coating over the cereal mixture, stirring as you go. Gently turn the mix from the bottom of the bowl up so every piece gets a light, even coating without crushing the cereal.
3 cups rice Chex cereal, 3 cups corn Chex cereal, 2 cups wheat Chex cereal, 1 1/2 cups mini pretzels, 1 cup pecan halves, 1/2 cup pumpkin seeds, 1/2 cup unsalted butter, 1/2 cup light brown sugar, 1/4 cup pure maple syrup, 2 teaspoons pure vanilla extract, 2 teaspoons pumpkin pie spice, 1 teaspoon ground cinnamon, 1/2 teaspoon fine sea salt
Pour the coated mixture onto the prepared baking sheet and spread it into an even layer. If the pan is crowded, divide between two baking sheets so the mix bakes instead of steaming.
Bake for 45–55 minutes, stirring every 15 minutes. Each time you stir, bring the edges toward the center and the center toward the edges. The mix is done when it feels dry to the touch and smells toasty and warmly spiced.
Remove the pan from the oven and let the Chex mix cool completely on the baking sheet, 30–45 minutes. As it cools, the coating will firm up and the cereal will become extra crisp.
When the mix is fully cool with no warmth remaining, stir in the cinnamon or white chocolate chips, M&M’s, caramel bits, and dried cranberries, if using. If the mix is even slightly warm, the chocolate may melt.
1 cup cinnamon chips or white chocolate chips, 1 cup fall-colored M&M’s or candy-coated chocolates, 1/2 cup caramel bits or chopped soft caramels, 1/2 cup dried cranberries
Transfer the Pumpkin Spice Chex Mix to a serving bowl, individual snack bags, or airtight containers. Seal promptly to keep it crisp and fresh.