Whatever you do, take sunscreen with you! And apply repeatedly.
I got a real bad sunburn in early May this year in Toronto while I had expected to return home with my lips blue from the cold. Nope. Coming from a Mediterranean country, I don't even have a fair complexion and I am really used to sun. Regular moisturizer (SPF 15) was not strong enough to protect my pre-tanned face.
As to your question: For one day, I second the Distillery District including one or three of Mill St.'s beers and the St. Lawrence market, but I would advise against going to one of the islands. I made that mistake on my first visit to TO which was a one-day flight stop-over. Yes, the islands are beautiful (and you can rent a bike if you have problems with walking), but it takes too much time, and to get to the ferry you somehow have to pass under the Gardiner Expressway which cuts the lake off from the city. One of the ugliest pieces of urban planning I have ever seen in my whole life. After that visit I decided that I would never go back to TO.
Most recently, I had to spend two weeks in TO, and I am now really falling for the city. Great neighborhoods, pretty good public transport (in addition to the subway and buses, they have these cute streetcars I learned to appreciate in Europe), diverse population, nice lake with the beautiful islands, tons of good food at fair prices, indie bookstores, alternative music.... I could go on for hours.
Try to get out of downtown and check out one of the neighborhoods, for example Danforth Av. (Chester or Pape subway station) which is close to Greektown with restaurants, cafes, and bars, and not to far from the Indian Bazaar already mentioned here.
Two slight drawbacks: I found the city's pace rather slow. A tad too slow for me. Um, actually, much too slow. And I was surprised that a lot of Torontonians in 'official' positions like subway personnel, ground staff at the airport etc. were far from the often described nice and polite Canadians. So if you got to TO, slow it down a bit. [This, I should add self-critically, could also have been a result of our interactions. I cannot count the times I was told not to be so impatient, to slow down, to let them finish (people were speaking so slowly, it drove me nuts)....]
*Warning*
Shopping: if you are not a Canadian resident, you can get taxes refunded -- the strip on Bloor
between Yonge and Avenue has a good range of upscale shops. Kensington market is more colourful.
This seems no longer be true. I bought books for more than CAN$ 300 and had to buy a new camera, only to learn at the airport that tax refunds were canceled about a year ago. (Could this be different for US-Americans due to NAFTA? I don't know, I am not an American.)
