Don't bite off more than is feasible. Take time to enjoy the people. They have the hugest welcoming hearts of anyone I have come across in the world, if you take the time to say hello. They have little patience for those who are in too much of a hurry, though.
Take a good spread of clothing. September can be wonderful and summer like, although always cooler from the close atlantic influence than central Canada is at the same time. It can also turn cool and rain, and you may find yourself a long way from somewhere to get warm.
Yes, don't plan to drive at night. I'm not kidding. Moose are a major issue.
I spent a week poking not more than 10km from David Whites' cabin where we holidayed two summers ago, outside of Lewsisporte, I did not feel short changed photographically staying so close to home.
Previously about 12 years ago I have taken in Gros Morne, St. Anthony's, and Lanse aux Meadows, and they are all most worth the trip.