If you hike, fall is one of the best times to visit the southwest. Crowds have thinned, the days are cooler, and small spots of fall color can light up the canyons even more. Frequent summer thunderstorms can become less frequent. The days are shorter. You have to make good use of your limited daylight, but it makes it easier to catch sunrises and sunsets and to enjoy the stars.
To take advantage of this, we visited Zion National Park in November 2015, spending a day and a half in the Zion Canyon section, a day in Kolob Canyons, a day in the Subway, and a beautiful morning on the Zion-Mount Carmel Highway.