Denver, and the front range sits at about a mile high. The air is a little thinner and is noticeable. Some tips are: Drink a lot of water & electrolytes, take an ibuprofen if you've still got a headache, moderate your activity if higher elevations aren't familiar to you, take an ibuprofen or two before bed to cope with the elevation, and consume alcohol more cautiously. Please consult with your doctor if you have concerns.
Yes. The sun is more intense at higher elevations, and Denver gets a lot of sun all year. I use websites like this to check the UV index throughout the day: https://uv.willyweather.com/co/denver-county/denver.html
Denver is typically quite moderate in the fall, but the weather can vary. It could be 75 degrees and sunny, or 35 degrees and storming. Please check the weather before you pack!