How are you getting to Missoula?
Do you adjust well to driving on the right side of the road? Expect to see snow on the road at least once during your trip.
Share a little more of your plan including the plan for the rental car. Those of us who live in the US, would incur a large drop off fee renting in Missoula and dropping off in Denver.

Going to the Sun Mountain Road closes on the 22nd

It could close earlier if a cold front passes through.

The Montana pictures were taken either going to and from Glacier or to Yellowstone. The are geo-tagged, which might help in establishing an itinerary should the sites interest you.

http://www.flickr.com/photos/mksfca/sets/72157623117942088/show/

Just remember that winter weather can start early, especially in the northern Rockies of Montana.

I would encourage you to read some of the trip reports posted on visits to the areas you mention. I think (and I mean this as a helpful suggestion not a criticism) that you might want to pose some specific questions about your trip planning.

I am thinking you could have an awesome vacation at this time.

Now, as has already been said, Glacier can be turning to winter unpredictably at this time. I've only been once, early September 2003, At that time, horrendous wildfires had been raging, and then the cold rains set in and put an end to them. Between the smoke/fire and then rain/fog we didn't see much, so I'll leave that to others.

Elsewhere - this would be a beautiful time to explore the Colorado Rockies. Last September, we spent a week in Durango and explored up to Silverton, Telluride, and nearby areas, finding absolutely gorgeous fall color.

I travel to Denver on business more frequently lately, and can say that the weather can vary wildly, thanks to the mountains and abrupt ending onto the plains. Tomorrow I'll go out to single digit temps and within a couple of days will have 50+ weather (fahrenheit).

So, hard to say. But, early fall is more stable weather-wise than late-spring/early-summer. Go for it!

Another note - our favorite vacations involve the mountains in the U.S. and Canada, and occur in September/October. Yellowstone is a favorite, to the extent that we are heading out there in a few days in the middle of winter.

By all means make sure you can spend time in Yellowstone and the Grand Tetons, thinking 10-14 days optimum for your schedule.

Thanks for your helpful comments, and the great pictures. It certainly makes us think this could be a good trip.
We would be hiring a car so I need to check about the one-way drop off charge. We (or rather my husband) is happy driving in the US - actually he prefers it to here!
We will put more detail together and then I will re-post.
Thanks again!

Labor Day, the last holiday 3 day weekend of the summer, falls on September 2 this year. I would plan to arrive on that date to start your trip. The kids will be back in school, the crowds will be less, and bookings easier all around. You are right to start in Glacier and move south. If you do run into snow along the way it will most probably not be a problem as you are taking a well traveled route between national parks.

A good place to start your research is NPS.gov. I would try to book lodging inside the parks. If you find it sold out for your dates be patient, check back often, and you may get a lucky (for you) cancellation.

As for route tips, I would suggest from Grand Teton NP that you head south through the Flaming Gorge entering Colorado at that point and continuing on to Rocky Mountain NP from the east before ending up in Denver.I just think it is a more interesting drive than across WY on 80.

There will be a rental car drop off fee but probably not enough to warent driving the hundreds of miles back to your starting point.

Look into carhire3000, they will rent from the majors and include all the insurance you need. Sometimes they can also get the dropoff fees waived but not always. Gets good reviews on TA.

The last two times we've been to Glacier National Park, we've stayed until about mid-September - a beautiful time to visit. Just be aware that a lot of the ranger led hikes usually end near or around Labor Day weekend if you're interested in hiking.

The most scenic part of the park is Many Glacier and I would recommend that's where you spend most of your time. You'll have more opportunity to see wildlife and if you plan to do any hiking, that's where you'll find the most popular trails, like Grinnell Glacier and Iceberg Lake. If you want to cut some miles off the Grinnell Glacier, as a lot of people do, take the boat ride across Swiftcurrent Lake and Lake Josephone - but make your reservations a few days in advance as that's one of the most popular hikes in the park.

http://www.hikinginglacier.com/grinnell-glacier.htm

Iceberg Lake is another really popular hike and usually has lots of hikers on the trail, but it also is known for a lot of bear activity. Be sure to check with the park service the day before you hike to make sure the trail is not closed due to bear activity.

http://www.hikinginglacier.com/iceberg-lake.htm

See if you can get in Many Glacier Lodge and ask for one of the remodeled rooms, if not, try Swiftcurrent Motor Inn.

I would also recommend spending 1-2 days on the west side at Lake McDonald Lodge and definitely take the boat ride on Lake Mcdonald.

Two lovely hikes on the west side are Trail of the Cedars and Avalanche Lake:

http://www.hikinginglacier.com/trail-of-the-cedars.htm

http://www.hikinginglacier.com/avalanche-lake.htm

We visited Aspen, Colorado once in early September and there was a big snowstorm.

Barblab, are you sending them south on US 191 to Flaming Gorge, then across through Steamboat to Granby on US 40, then into the park from the west on US 34?

Could be a beautiful trip. I 80 would not be.

I would suggest from Grand Teton NP that you head south through the Flaming Gorge entering Colorado at that point and continuing on to Rocky Mountain NP from the east before ending up in Denver.I just think it is a more interesting drive than across WY on 80.

An alternative is to go to Dinosaur National Monument in Utah, driving through the Uinta range, and then go east through Rocky Mountain National Park to cross the Continental Divide (assuming that weather conditions permit it).

http://www.flickr.com/photos/mksfca/4810625433/in/set-72157623176227912

Not knowledgeable about farther west, but typically the third week in September is prime Aspen leaves turning in Colorado. I would make a stop bewteen Aspen and Crested Butte. I think it's the largest continuous aspen forest. or at least go to the Maroon Bells outside of Aspen. The colors should be gorgeous.