Forest Park
Okay, so this one is not quite hidden, and not quite on the grid, but if you're willing to make the trek (it's on the Brooklyn-Queens county line), Forest Park has over 500 acres of open space, 165 of which are trees. If you're feeling oxygen-deprived and want to watch sunlight filter down through leaves, this park is your best option. You can hike the trails or hire a horse to do your walking for you if you're not a pleb.
Forest Park, 8001 Myrtle Ave, Flushing, NY
[Photo via @tailored.story]