Spray a 13 by 9-inch baking pan with nonstick cooking spray.
Melt butter in a large pot over medium heat. Quickly stir in the marshmallows and cook until completely melted. Remove from heat.
Stir in cereal, and dried fruit and seeds if using, until well-coated and combined. Spray a rubber spatula or wooden spoon with nonstick cooking spray and press mixture evenly into pan. Cool and cut into 2 by 2-inch squares or use cookie cutters to cut into playful shapes. Pack in an air-tight container.