Growing up in Detroit, most stores accepted Canadian money, until around the late 1960s when the value of the Canadian dollar dropped substantially.
Canadian is worth a lot more than a peso. Canadian coins are still common.
By my experiences in visiting north central Minnesota, you're quite likely to find a Canadian coin or two in your change.
My US dollars have been accepted in stores as far south from the border as Villahermosa and Mexico City, as far north as Saskatoon, Regina and Winnipeg, as far east as Kuwait. I have also been in stores in Minot and Bismarck, ND that will take Canadian dollars. Why is a pizzeria anywhere accepting foreign currency a bad thing. Maybe we should all focus on the real issue of getting people to come here legally to spend their foreign currency.