Steve, your Sept 20-21 could be a bulls-eye for peak aspen colors. I think Odessa Lake might be my favorite campsite in the park. It's easier to reach via Bear Lake than from the Fern Lake trailhead. Another scenic campsite is Ouzel Lake. An easy one, at a lower elevation, is the 2-mile hike to the Peregrine campsite. That area is loaded with aspen, and it's right next to a nice creek. In addition, there's a nice waterfall you could hike up to after you set up your tent/treat your water to fill out the afternoon.
Edit to add: No fires are allowed at the vast majority of RMNP campsites, so we just use canister stoves. Also, bear canisters are required everywhere in the Park. You should plan that it could get down to near freezing at night (probably low-mid 40s though), but the day's highs could well be in the 70s.