Been a long time since i’ve been to Banff (1992), and when i went it was the middle of winter, and i didn’t do much except ski. If you’re actually staying in the town of Banff, a trip out to Lake Louise is definitely worth the effort. It’s beautiful out there.
Now, i might be wrong about this, because as i said it’s been quite a while since i was there, but i’m not sure how much benefit you will get from going in “foliage season.” If i remember correctly, most of the trees in the region are pine, fir and spruce trees that tend to stay green most of the year round. You certainly won’t get the riot of colour that you might expect from a New England fall.
I’d be happy to be corrected on this by someone with a better knowledge of the area, but i lived in western Canada for a couple of years, and made trips in BC and Alberta, and i never saw the level of fall colour that i’ve seen in the eastern part of the continent.
Anyway, whenever you go, the Canadian Rockies are a beautiful place. Hope you have fun.