I live right in the city and think that we have some pretty good shopping (everything from mainstream to blow your budget!). I'm not sure how you will be getting around when visiting us, but I'm going to give you some basic directions assuming you are going to use our transit system at least a little (which I would recommend while in the core, cause parking is sucky times)
Queen Street West is great for boutiques, as mentioned (Walk west from the Queen Street Subway Station). Also the Yorkville ares has some really unique stores (Bay subway station), and some luxury chains, but is a pricy pricy area. It also has one of Canada's only Anthropologie stores, but if you live near one in the States, that might not be a draw.Vaughn Mills and Dixie Outlet are great for deals (although I find them both really hit or miss in terms of stock and not always worth the time), but if you're looking for shopping that's accessible from Toronto's downtown, then you may want to check out Yorkdale shopping centre (it has its own subway stop, really easy to find!) for high end stuff like Holt Renfrew, and major chains like H&M and Zara. Right downtown, there's the Eaton Centre (accessible from Dundas or Queen subway stations), another mall full of mainstream stores, but it has a good RW&Co. (a Canadian exclusive) along with Smart Set and Jacob (more Canadian exclusives, more casual but good stuff).
And if you want to check out the Joe Fresh stuff (which I'd totally recommend, don't let the fact that it is sold in grocery stores scare you!), there is a stand alone store in Toronto at the St. Clair West subway station. There is one on Queen's Quay (near the waterfront) as well.
Have fun!!!!!