After we cross the border, our first stop is a Canadian bank. We get $300 Canadian. We use public campgrounds and the majority take only cash in their drop box. Some small restaurants only take cash. Also, laundramats, small grocery items, emergency fuel if no credit card option, etc. If you have any left before crossing back into the U.S., spend it on fuel or a restaurant. $300 will be plenty for the whole summer.
We didn't carry more than $300 in U.S. cash either. You can always get more at an ATM.
Visa is widely accepted, not Discover or American Express.