I would take out cash from an ATM, that's the easiest and simplest option. Some places will take US cash, especially at Niagara Falls, but the further you go from the border and tourist areas I would doubt it. Certainly the exchange rate will be a lot worse that what you'd get from taking Canadian cash from an ATM, usually by a significant percentage.
A simple way of putting it in perspective is if you went to a store in California and tried to pay with Canadian dollars or British Pounds, how far would you get and what sort of exchange rate would you get? Not great I expect.