I used to live in MV--I taught and lived at the high school. I think the best time is the last week of October/First week of November--even second week of November. This probably sounds too late for the fall, but it's when all the colors are out. The yellow cottonwoods in the canyons surrounding the valley are magnificient, and in addition to the valley itself, there are alot of really great canyons within an hour - valley of the gods, mexican hat, etc. Who knows, if you're lucky you might even get a morning or two of intense fog!