I agree driving the Icefield Parkway from Banff/Lake Louise to Jasper is your best bet. (I´ve taken that route a dozen times and it´s always been a fascinating trip)
Prepare for any sort of weather, it should be pretty nice/warm in the afternoon, but there´s also a chance for temperatures to drop below 0 at night, personally September is my favourite time of the year in the Rockies.

I´d definitely recommend booking a hotel in advance as the Rockies are still pretty busy in September (though not as bad as July/August).