Just a short 15 minute ferry ride away from the mainland lies the Toronto Islands, a lovely escape from the madness of city life.
Pretty easy to get to using public transportation. You'll want to arrive at the Jack Layton Ferry Terminal before 10 am or after 6pm to avoid the crowds during the Summer or Fall seasons (LINK- http://www1.toronto.ca/wps/portal/contentonly?vgnextoid=3690dada600f0410VgnVCM10000071d60f89RCRD). It costs $7.50 for the ride to the island and back; you can catch the ferry ride back at any time. The ferry ride is very relaxing, it feels like you're gliding over the water. You get such a beautiful panoramic view of the Toronto skyline from the deck of the ferry. Once you get off make sure to pick up one of the maps at the informational booth to the left of the Lions Club Pavillion (see pic).
For new comers I would suggest to take the ferry to Centre Island so you can take the Toronto Island Tram Tour. First tour boards at 12 noon. If you take an early ferry you'll have enough time to take tons of pictures of the flora, fauna and the many different features on Centre Island before the start of the tour. The tram will take you around the island from Hanlan's Point to Ward island and back in about 30 minutes (LINK - http://www1.toronto.ca/wps/portal/contentonly?vgnextoid=ea5f249e30b0f410VgnVCM10000071d60f89RCRD&vgnextchannel=34e9dada600f0410VgnVCM10000071d60f89RCRD). It's a relaxing low stress way to view the entire island. As an added benefit, you also get historical and informational tidbits from the "tour guide" (the tour guide is actually a recording that the bus driver plays).
Tons of things to do - play in the sand at Manitou beach, ride/paddle a boat through the waterways between the islands, enjoy a walk/jog/bike through the park, play a game of disc (frisbee) golf, visit the animals at Far Enough Farm, ride the rides at Centreville Amusement Park. Don't rush around, soak up some sun and enjoy the day.