Most definitely worth the visit. Might be a bit cold in late September, but driving the Going to the Sun Road is certainly a pleasure in itself. There are many side stops that you can pull over just to enjoy the view. These stops are all well paved. The Trail of the Cedars and Lake McDonald Lodge is also wheelchair accessible.
My siblings and I recently visited and my 80 yr. old father chose to stay behind. If it's worth it to you to enjoy the sites from the car and it's not too much of a delay then maybe??? There's no internet for much of the drive. Personally I'd say no unless it's the only time you'll have the chance. I doubt it would be worth it for mum unless her vision is vastly better than most 80 yr. olds.
There are a fsir number of paved trails and boardwalks avalable at Glacier. There appeared to me to have been a lot done to make more of the park available to wheelchairs. I you can afford it you can arrange a tour on the red busses, or take her on the going to the sun road yourself.
The drive going to the sun road is beautiful! Most of the pictures of this park which I took was driving. There are some small trails which are wheelchair accessible!
If you stay only in car with window open its worth it. Most of the view spots are wheelchair accessible.
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