As Robyn mentioned, the weather in or near the mountains is hard to predict in the fall. The deciduous trees will likely still be golden, and last year (my first in Boulder) we didn't get snow in Boulder itself until the week of Halloween. However, the higher elevations are only snow-free for a short time in the summer. Don't plan to do any really high hikes (12,000ft+) unless you know how to travel over snow. There's usually a lot of beautiful trails still open in the lower areas in the fall and year round. Around town, there's Mt. Sanitas, Flagstaff Mtn, Green Mtn, and Bear Mtn, depending on how fit you're feeling. North is Rocky Mountain National Park (variety of terrain, lots of wildlife). South is Garden of the Gods, Manitou Springs cave dwellings, and Cave of the Winds (all near Colorado Springs and open well into the fall, if not year round). West is a great trail to Hanging Lake (& Bridal Veil Falls) near Glenwood Springs. There's lots of state parks around too. Eldorado Canyon (W) and Roxborough (S) state parks are easily accessible and offer some great views with little to no effort. If you're into sports, there's a Colorado Buffaloes football game on Oct 2. If you like museums, there's a ton in Denver, not so many in Boulder. There's restaurants galore, and some small theatre/ballet productions, not sure what's playing when. I hope that you enjoy your visit--I've only been in Boulder a year and find I'm still discovering new things to see and do here!
