At the beginning of October the fall colors are finally starting to appear in Zion National Park. The first areas to see them are those of higher altitude, east of the tunnel. Once you exit the tunnel on the east side, drive a bit until you find a good parking spot to go down on the right side of the road. For example, there is such a spot right before the second (short) tunnel. Park there, and go down to the wash (it’s called Clear Creek, although there is hardly ever any water there, only after a strong rain).
You can walk the wash for quite a while in either direction, and you would be surprised with what you see. It’s a good place to explore Zion any time of the year, especially if you want to get away form the crowds: you would probably not meet another person there.
The beginning of October is usually the best time to see the fall colors in Clear Creek.