Are you going all the way to TO just for one day? Or is this a stop-over type thing?
TTC is super easy (and cheap!) from the airport to downtown, so that's probably your best bet. If it's just for one day, however, you may want to consider the Airport Express:
http://www.torontoairportexpress.com/ Not sure if the extra $ is worth it though -- it may or may not be faster given the time that you'll be travelling (basically, rush hour both ways).
P.S. Don't forget to buy a hot dog from a street vendor while you're there
