Most places will accept US money but be aware of two things:

1. The exchange rate may not be great.

2. By law, change can only be given in Canadian money. (This includes on the return trip in the stores/restaurants/duty free areas after you have cleared US Customs & Immigration and are awaiting your flight home.)

If you have a Bank of America ATM card, use the BanqueScotia/Scotiabank ATM's as they have a reciprocal arrangement that neither BofA or Scotia will charge those irritating $2-5 fees!

As well, tips are often automatically added into restaurant/bar bills in Quebec, and it will probably be shown in French as pourboire.

Great city! Have fun!