On Parks Day this July 17, Parks Canada will host an open-door event at all national parks, national historic sites, and national marine conservation areas.
Go to this link for ones close to you:
In Toronto for most of the attractions you will be driving to the Hamilton area, Niagara area or East of the city to find your park/historic site--but make a day of it and have fun! Makes for a great family outing.
Note: Fort York is run by the city, not by parks Canada even though it is a "National Historic site"
To find a Parks Canada national park, historic site or marine conservation area near you, visit the Parks Canada website.