Drive through the National Bison Range N of Missoula. Spend about 2 hours on the long loop. The southern 2/3rd of Flathead Lake has a lot more character than the part you will see from the road on the North end.
Glacier is busy this time of year. If your going to G Falls and taking the Sun Highway, you will be on the inside lane. That is a good plan for first timers.
Plan on daytime temperatures from 90 F to 40 F in that time range. When I lived on that side we occasionally had frost by Labor Day but usually more by the 2nd or 3rd week of September. Expect mostly dry weather. We are burning up currently I visibility in SW MT is around 4-5 miles. It will take snow to put out the fires we have now. What you see will be different and pretty, but may be effected by smoke.
If you don't have reservations get them now. It can be impossible to stay in some places on a spur of the moment. If I were going I would consider the drive North from Missoula on 93, stay in the Flathead and see Glacier by going E from Columbia Falls, stopping at Goat Haunt, then entering the Park at the East Entrance. Go North to Many Glacier and Two Medicine then go over the Sun Road. If you want to avoid crowds in scenery and have the best chance of seeing any wildlife, be on the road at 6 AM. Sunset will be around 8:15 then. Glacier has some short hikes of less than 1/2 mile. Have a great trip.
