" but the big Canadian banks give pretty good exchange rates "
Not this sadly.
Not this sadly.
If you want to convert your funds at a "Big" Canadian bank, please do so at Tangerine, which has the best rate I found (they take 1.45%) - otherwise, there are plenty of other suggestions in this thread that could help. The best rate will most certainly be with Interactive Brokers (max(0.01%, $2.50)), but the account set up is a bit tedious and they have a few restrictions.
That said, it's much easier to convert USD to CAD in Canada, then CAD to USD in the US. I had an account at a small regional bank and if you were converting less than $100 of CAD, you'd be lucky if you got 2/3rds of the actual exchange rate.