Sentinel Pass and Larch Valley are an absolute must do in September. The view from the top of Sentinel Pass is well worth the hike up. I'd suggest bringing a snack to enjoy at the top, because you will definitely want to spend some time up there.
To do this hike you need to park at Moraine lake. I would suggest doing this on a weekday if possible and arrive early for a better chance of the parking lot having spaces available. If it's full you will have to take a shuttle up and back down to your car. The shuttle area is well marked off the highway before you hit Lake Louise and costs approx 20$ per person.
Sentinel Pass is a 10.9 km hike with a 792 Meter Elevation Gain. (Approx 5 hours) Dogs are allowed on a leash. If Sentinel seems a little steep for your tastes... The hike to Larch Valley is only 4.3 KM with a 535 Meter elevation gain (3.5 - 4 hours). You can decide once you arrive there if you have the energy left to venture up the switchbacks of Sentinel pass. If you don't it won't ruin your day as Larch Valley is gorgeous to spend time in.
The pictures without snow are from September 10 2017.
Pictures with snow are from September 27th 2019.
I definitely recommend crampons and poles if there's snow... also maybe not wearing shorts like the guy from Sweden did.