If you enjoy photography or just the aesthetics of Fall, the mountain valleys and grasslands of S-SE Idaho offer a beautiful desiduous display of Fall color. The harvest season makes for some great farmer's market shopping in those locales, too, so you can store up a winter stash of fresh goods, just like the squirrels.
Also, as boi2socal noted, 'tis the season for hiking, biking, and pretty much all the other stuff that wasn't so tempting or enjoyable in the summer heat. Spring and Summer make for great water recreation, while Fall makes for great land recreation.
With that, I think I pretty much covered the general Fall activities for both old and young folks.