Most of our activities occurred in the morning, as the afternoons were reserved for the all-important nap. We spent the first day simply testing out the subway and touring around downtown. My almost four-year-old was fascinated by the "trains" and wanted to ride the subway all day. We arrived downtown, climbed the stairs to the street-level and he was overwhelmed by the noise and the people. He actually physically covered his ears, closed his eyes and curled up into a ball at the top of the subway stairs. But he quickly got over it and was accustomed to the hustle and bustle of downtown Toronto in a matter of acouple of hours (unlike his mom...). A word of warning, however: I found the subway system less than stroller-friendly. There were lots of stairs all the time, lots of people wanting to go fast all the time, and when we did find an elevator, it reeked so badly of urine that I was gagging during the short ride to the next level. The people were friendly, though, and always offered their seat when it was busy to me and my two young children other people offered to carry strollers up or down the stairs. That was very nice otherwise, I think we would still be in the subway tunnels somewhere.
We also visited Centre Island, which was excellent. We combined a subway ride, a streetcar ride, and a boat ride all in one day! Centre Island has a good splash park and a petting zoo as well. There are lots of rides too, but they weren't opened when we were there. We spent a day at the Toronto Zoo, which was also great. We had to rent a car to get there, and the zoo itself is quite pricey, but it was a great day. We saw lots of animals and walked a great deal and only managed to see less than a quarter of the complex. If I lived in Toronto, I would certainly buy the season pass - it seems like the only way to see the whole zoo.
And we traveled North to cottage country for an overnight visit, which was excellent. The kids got to run around without the threat of cars or people, and we got to relax in wonderful hospitality. We had an excellent meal in Bancroft at The Granite - check it out if you are headed that direction.