Go to the TRCA link above that Ranger Jay recommends.
They have two campgrounds that are RIGHT IN THE CITY.
Glen Rouge, in a very lovely setting along the river near the Toronto Zoo, has about 125 sites, but only water and electric.
Indian Line, in the northern part of the city, is bigger and has full service sites. I haven't seen it.
You can reach the heart of the city from each one in about 35 minutes driving.
Puplic Transportation serves each site as well and could be the easiest way to get DOWNTOWN in the Big City.