is not predictable. Usually the aspen turn in late Sept. or in Oct. but if we get a wind or snowstorm the leaves will be gone soon after. Sept. is usually warm and summerlike but we are always grateful that nights are cool no matter the day temps. You'll want to "acclimate" by staying in Boulder for a day or two if you plan to head to Estes Park and Rocky Mountain National Park (to see the elk bugling and the bighorns), the narrow gauge train that runs between Georgetown and Silver Plume, or even Garden of the Gods and Pike's Peak in Colorado Springs. There's lots to do in Denver too: the Art Museum, the beautiful Botanic Gardens, the great Nature and Science Museum and Zoo, and the Rockies will be still be trying to at least win the West Division.
In Boulder you may want to see the University of Colorado campus, the rock climbers in Eldorado Canyon, Pearl Street Mall, Chautauqua (our wonderful music festival happens between June and August), Boulder and Left Hand Canyons to Nederland, Jamestown, Gold Hill, Peak to Peak Highway, or just walk or bike the many trails which are in and around the city and foothills.
I'm sorry Boulder is no longer the small town it was when I came to Colorado in 1975; we don't enjoy the development and traffic but it's still a great place to live and visit, especially in the fall.
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