High Park is a fantastic park in the middle of Toronto. I've been here a couple of times so far, mostly for a walk and some geocaching, and I've really enjoyed my time.
It has plenty of amenities, from baseball fields to hiking trails, and a snack bar. There's a shuttle that goes around the park if needed, although both times I've been there it hasn't yet been running.
I really enjoyed the contrasts between the wilder areas and the more civilized ones. This park is pretty gigantic, so there are many reasons to make return visits.