Autumn around Flagstaff usually happens the first to the tenth of October, depending on the year. Lockett Meadow is one of my favorite places because of the aspens. The road to Snowbowl is fine work as well — aspens along the climb. One more area that should be checked is the dirt road around the western part of the San Francisco Peaks.
This is a fall portfolio, not a year-round Flagstaff survey. The aspens are the subject. The journal has other autumn notes from Zion and the North Rim if you are planning a loop.