A trip in September is still very enjoyable. Just have to adjust to shorter daylight and cooler evenings and fresh dew in the mornings. Warm sleepwear may need to be added to your pack.  You may find some colours appear in the woods and fishing for lake trout becomes easier.
We have a few groups paddling and camping in the park this month. I myself will be going in soon for 4 days for a quick project. Our biologist crew is flying out today for a week to assess caribou habitat in the recent burn as well as evaluate some campsites found within the burn.
There's still blueberries along some trails.  At this time, moose are moving along in search and beavers are busy beefing up their feedbeds.