Changing Money

ANY CANADIAN CASINO

If you stay at many of the provincial parks or similar small campgrounds (especially in the less populated provinces/regions), cash is very, very useful for the iron rangers.

Another vote for ATMs. It makes a lot of sense to do a few larger withdrawals than many small ones as often the fees are per transaction rather than per dollar.

Do you really need cash? I would try to go through on just VISA/MC/AMEX.
We did fine for 4 days last Summer visiting every eatery, fuel station and tourist trap we could find. No cash.

Check with your home bank to find out what additional charges/fees are added to using your debit and credit cards for purchases in foreign currency like C$, and if there are per transaction fees, as well.
There aren't a lot of places in Canada that don't take one or the other of these payment methods, so you can probably reduce the amount of cash you bring, and just bring plastic. You'll get whatever the VISA/Mastercard/ATM/Debit card foreign currency exchange rate in effect on the date/time the transaction is posted to your account.
We use 2 no fee Canadian based credit cards that also have a 0% foreign currency exchange rate (most Canadian credit cards charge 2.5% of the purchase value to do the forex conversion), and so buy pretty much everything using them when we travel in the US. We get whatever the VISA/MC exchange rate of the day happens to be at time of purchase.
We might have some cash if we're camping in places that use iron rangers. If we need more cash, we bite the bullet and go to a local ATM and pay the fee.

I always just use an ATM. I get enough cash to last a week or so at a time, then pay for everything in cash. That way I miminimize the fees and increase the convenience as I can get more cash wherever I'm at.

You may check with your bank and see if they have a "sister bank" in Canada. Some do and will waive certain fees if you use that bank's ATMs.

There is no best place really. Some use their Visa cards because they get points for using it, and that can take a bit of a bite out of the charge to exchange it. Most banks near the border will trade cash for you. We find it's best to have both available, but now we have US accounts so use them exclusively. Our US credit cards get used there and paid with US funds from US investments.

Withdraw money from an ATM with your debit card especially if your bank does not charge fees. Use your credit card whenever possible (also if they don't charge fees). If your bank(s) do charge fees it may be more economical to get a card that doesn't and use it for your trip.