This is a great subject for frugal field trips.
-Take a nature hike and observe the changing leaves, the squirrels foraging for nuts, and other signs of fall.
-Go apple and/or pumpkin picking. See if the farm offers tours. We had a great lesson a few years ago on the benefits of bees to apple farms. We also learned the best way to pick apples without harming the bud.
-Go on a hay ride.
-Visit a farm that owns a cider press and learn how cider is made.
-Rake leaves for an elderly family member or neighbor.