I really liked the North Market (farmer's market) more than the crowded South Market (prepared foods, two floors, I suggest strategizing before you get there). The North Market is only open on weekends though, and if you've only got a carryon you won't be able to carry a jar of the delectable hot sauce they sell there (or any of the specially flavored mustards in the South Market).
Compared to earlier reviews here, I felt that the cheese and meat vendors weren't particularly friendly about giving samples out...so I walked away empty handed and got a few egg tarts and a sandwich from Carousel. And compared to markets in other cities, this isn't necessarily a go-to spot, but if you're around on the weekend, it is a staple of Toronto tourism, and who can turn down legit Canadian (=peameal) bacon?