My wife is Canadian, and we visit her family often. Our experience is that almost everyplace within a 100 miles or so of the boarder or near any tourist area will take US dollars, apply an exchange rate which varies from place to place, and give you change in Canadian dollars. The best exchange rates - mat least in Ontario - always seem to be at the Beer Store.
We have a credit card and a debit card which we can use in Canada without any upcharge - shop around and you can find them. The bank exchange rates change daily. We use the debit card at Canadian ATM's to get Canadian dollars, and put fuel and other major purchases on the credit card.