What’s the weather like in late Sept and what’s to see? Which guppies live there etc etc.
hey Thap 
our relatives have lived there for 18 years - it is a very nice place to live - September is one of those months where it could be 85 degrees outside - and then 10 mins later it is raining hard.
Still warm from the summer but nice cold wind from the mountians - by October-Novemeber - the Aspen trees start turning a beautiful gold and the air is so crisp - it really is a great experience living in Colorado. If taxes weren’t so high - and if everything wasn’t an hour drive through the moutains - we would want to move back.
Overall - Colorado is a dope place to live. 
If all else fails, there's always aj as backup.
Thap Bhaijaan, Colorado is not England, or Egypt for that matter, where the weather of the whole state is the same at any given time of the year. So tell us where you plan to visit & you can get a little better detail. Up in mountains it's always cold, in the front range it's usually extreme - freezing at nights but hot by afternoon and in the eastern planes it's windy for sure ... sometimes cold sometimes hot ... depending on the weather system.
If you were wondering what clothes to bring, I would suggest bring everything. As the natives say, they have seen snowfall in July ... so September should get it's fair share. This doesn't mean you won't have days were it will go up in the 90s.
Most of the things to do are in the west .. Denver is ok, but only if you are into cities. Otherwise it’s like any other big city in USA .. probably more polluted. Starting from the front range [Boulder, Fort Collins, Colorado Springs] you have a whole lot of things to do. You can go horse back riding in the Rockies [though it might not be possible in late September], hiking, skiing [both cross country & resort kind ], snow shoeing, rock climbing etc. It’s all there ..
Oh there is also a train that goes from Denver to Salt Lake City [Utah] ... and it’s supposed to be a heavenly scenic route.
Don't forget Aspen!
If they were to go to Utah, they might as well go to Las Vegas and California too. Or travel down to the border of Mexico.
Check out the Mile High Club, and the local steak houses. There was one where they had a law of not wearing ties. I walked in once with my suit on and they chopped off my tie :)
Real coboy stuff over there.