You may find some businesses, especially close to the border, accept US currency but at par. That means they will exchange at $1 CDN for $1 USD. With the exchange rate of $1 USD to $1.30 CDN that is a very poor rate of exchange. Better to just go to an ATM in Canada and withdraw Canadian dollars.
Be sure to tell your credit card companies and banks where you have ATM's that you will be traveling in Canada or they will deny all transactions.