I was in Whistler with my wife & son a few months back and we just had to suck up the charges, but what struck me was the fact I didn't really need a car in Whistler.

We walked, we rented cycles and the car didn't move for the duration of our stay (3 nights IIRC).

Avis have an office right in town - have you looked at doing two one-way rentals (I assume you're not driving from Chicago...)

And on a similar vein you could always take the train or coach up to Whistler from Vancouver.