Even as a Toronto local I love coming to the market on the weekend to stock up on groceries. It can get a little crowded and touristy, but nothing beats a Saturday morning walk around and farmers market visit.
Visiting a few times will help give you a better idea of who has deals and who is trying to cash in, but stopping in for cheese and peameal bacon is a must! Don't miss out on Kozliks mustards (amazing maple is my favourite).
Also, the farmers market on Saturdays across the street is the place to go for fresh veggies - show up later in the morning around noon for better deals once the vendors try to distribute the last of their produce!