I love this park for exercise and just relaxing in general. I always thought of Toronto as the "city" with lack of green space, but High Park defies all odds as it has plenty of that and very large. They have some great areas to walk around, can see the cherry blossoms (as many have mentioned) and a little zoo as well.
While there's plenty of people, I still find it serene enough to relax and read a book or do some self reflection.
Overall, I think it's a great park, but the only reason it loses a star is because I tend to get lost a lot. :-(