I must disagree with the review below and say that High Park is no Central Park. That is in no way meant to be disparaging and if anything speaks much more to my love of Central Park than anything, because High Park is a wonderful oasis on Toronto's west side.
Basically if you enjoy being outdoors then it's impossible not to like this park. The park has a labyrinth of trails, which range from the straight wide paths in the off leash area to the small winding paths that circumnavigate the whole park and seemingly lead to nowhere yet are connected to a number of smaller nondescript entrances/exits; another unique feature.
There is a modest, laid back zoo as well as Grenadier Pond and a number of streams and ponds. I remember skating on Grenadier Pond as a kid but I think I saw signs that indicated that that was no longer possible. If that is the case it's too bad because it's a fantastic place to skate.
If you get hungry while your mid-park you can always hit up the Grenadier Cafe. It has very reasonable hours and plenty of options.