American Visa/MasterCard will work for many purchases, but Canadian and American debit don’t cross over. Also of note, not all exhibitors/artists will take credit/debit. It is always a good idea to keep some Canadian cash on hand.
In a pinch, many Canadian retailers will accept US dollar bills, however they usually accept the bills at face value (no conversion), or convert at a worse rate than a currency exchange / bank.
About Canadian currency: There are no bills smaller than a five. We have a one dollar coin referred to universally as the Loonie and a two dollar coin referred to as a Toonie. This is totally normal and if you laugh when we talk about loonies and toonies we will look at you weird.