What sort of things are you into buddy? You single? Married? Kids? Looking?
You into nightlife? Are you looking to catch a show? You into bars? Pubs? What kind? You on a tight budget? Toronto's got something for everyone...literally.
In answer to your question -- Visa or Mastercard are the best. Most places take AmEx, but not everywhere if you stray from tourist spots. I don't know anyone with a Discover card, so I'm not sure about that one.
Toronto is a great place. September weather is usually pretty good.
Drop any of us YYZers an email if you want. I'm sure any of us would be happy to help you out and answer any questions you may have. You can also check out this link - http://www.toronto.com/
Enjoy your trip.