I assume Whistler is beautiful anytime of year
 

May want to add a day or two there in Sept. I know it's a ski destination but is it a good destination any time of year if you like the mountains?

Well, I don't know if I'd want to spend a pre-ski season November in Whistler but September - definitely yes. Kids are back in school, weather is still Indian summer nice. Walk, hike, cycle, zip-trek, go up to the mountain peeks, eat and drink well... Definitely go.

The bike park is open until oct.There may be the big thanksgiving sale also that month.good prices on ski and snowboard stuff..
in nov. the slopes open up earily sometimes and the prices are really good.You might even get to see a bear or two

The drive to and from Whistler is worth seeing in the better weather, so yes, go for it!

Yes, but if it's raining or if there are low clouds, then you're really not going to see the scenery, and in that case, I don't think it's worth it. But if it's sunny or if there are high clouds (high enough that you can still see the mountains), then it's worth it. Just took family there for the first time this past Saturday and they were blown away by Whistler. Not by the village, but by taking the gondola to the very top. It was like being in a different world, especially when you walk away from the gondola station (The Roundhouse) and walk towards Harmony Lake. You feel like you could be in the Alps, as it's all glacial valleys and alpine forest.