In my experience, most Canadian businesses in major population centers, especially tourist areas near the border, are happy to accept US currency. Though they probably stiff you on the exchange rate.
That’s been my experience, and it’s not even an extortionate exchange rate, it’s just a not-very-good exchange rate. Now… trying to change dollars to euros at the airport (in the US or in Germany, the two places I’ve priced it recently) is truly extortionate.
The proprietor did not seem to speak much English, it took quite a bit of pantomime to convey that the reader wasn’t working. I showed him my American cash, and he shook his head, “no.” So I said, “ATM?” He nodded and pointed across the street. And that’s how the tiger got its stripes I paid for dinner.