Denver may not be cold, but then again, it might be. The temperature swings in Denver can be dramatic, and October is a good time for it to happen. I’ve been in Boulder, CO in mid-May and Mid-September and it snowed a few inches. If you go into the Colorado Rockies, be prepared for snow in October. The average snowfall at Yosemite in October is nothing. There is a little snow in November. But the farther north you go in the Sierra Nevadas, the more snow you will encounter. Squaw Valley has snow already. But if you stay to the south, you can avoid snow for now. Southern Utah could be a great place to sight see.