I am travelling three months from France to Waterloo.
What is the optimum way to pay for:
small amounts (groceries, Telecom, etc),
large amounts (house rental, local trips, etc).
Should I use
For the last two options:
How to get in the cash?
A local bank account and debit card would serve you very well. In Ontario cash is almost unnecessary because so many places take debit. We can also email each other money using Interac, which is part of the debit system. Most people I know email the rent to their landlord, for example. Everything I used to write cheques for (paying my farmer for veggies, donating to charity) I now email money for.
Compared to the US, you will not be able to use a credit card as much as you might expect to. Convenience stores, for example, will take debit but not credit. Restaurants and hotels and large stores will all take both. Debit can be awkward at places that want deposits (hotels, renting a car) so I recommend having a credit card as well. You can use your foreign one, but you'll be paying currency conversion fees. If you can get a local one when you open the bank account, that would be good.
As to whether you can open that account, it might be a good idea to contact a bank and ask. You might need to be a resident.
Getting the money into your account should be simple with a service like TransferWise. Your French account sends money to yourself (using your email address) and you deposit it into your Canadian account. You can reverse the process if there's money left over when you leave.
I spent 2 1/2 months in Waterloo in 2017, visiting from the USA. In general, I found that my US credit card served me well for almost everything I needed to do: groceries, restaurants, laundromats, day trips to Toronto, etc. I also used my US ATM card to withdraw a supply of cash a few times during the trip, though I found that I didn't need it all that often.
Pace Kate Gregory's answer, I don't recall running into any stores that would only take debit cards, though I don't doubt that they exist. The only hassle involved was the fact that most US credit cards require a signature for many purchases (chip & signature), while most Canadian credit cards do not (chip & PIN). Coming from France, I suspect that this will not be an issue for you.
Before my visit, I sought out and applied for a US card that advertised itself as having no foreign transaction fees to use on the trip. This said, I'm not sure that this really saved me any money, as I didn't actually check into the exchange rate the card offered, and I have a sneaking suspicion that the "foreign transaction fees" were just effectively rolled into the exchange rate I was given.
This said, my rent & utilities during were paid for by my hosts (Perimeter Institute), so I don't know whether credit cards would have been useful there or whether I would have required an Interac card. I also ran into one awkward situation where I paid for a friend's ticket to a hockey game, and she attempted to pay me back with an e-mail transfer (as mentioned by Kate Gregory) that I had no way of accepting. She paid me back with some cash instead, but I remember it being a bit of a hassle for her to "undo" the transfer. This probably had more to do with the level of customer service available at her bank, though.
