Which part of September? The last week of September is unreal with the aspens, as mentioned Kebler Pass has some unbelievable views. For fall colors, I'd also recommend:
- Dark Canyon Trail (near Horse Ranch Park). Easy hike, great views and an amazing overlook about 1.5 miles in.
- The Dyke- either the trail, or the main overlook off Kebler Pass.
- Swing around the backside (south east side) of Mt. Crested Butte and look down/up the East River Valley. There's a short trail from the TH you can walk easily too.
- Drive up towards Gothic, north of the ski area. Lots of awesome overlooks of the East River and up towards Aspen.
- If Snodgrass is open (it's on private property, so it usually closes in Aug/Sept), do this hike. It's a little steep at the beginning, but is worth it.
If you're going in early September, my choice of trails would change since the aspens won't be changing yet. In that case, I recommend:
- Paradise Divide- rough drive, but you can get good sunset shots from the parking lot.
- Yule Pass- easy 4 mile r/t hike with great views from the same area as Paradise Divide.
- Drive up and explore around Gothic, a small town a few miles up from Mt. CB.
- Dark Canyon Trail is still amazing, regardless of the season.
- The Lower Loop is a great option right next to town, good views of the Slate River and Mt. CB.
I'm happy to help out with some more suggestions if you have specific questions... I go to CB at least 2-3 times a year. Search my website's Trip Reports section- there are a handful from the last two years from Crested Butte. IMO, it's the best mountain town in Colorado, you'll love it!