The most densely populated urban areas are where a vast majority of above ground parking garages are found. These same densely populated urban areas also happen to be the places that are hurting the most for green space.
Why not build a superficial, extra floor on existing parking garages to include grass, trees, and bodies of water (fountains, reflection pools, etc.). Parking garages are built to carry weight, lots of weight, so I doubt that there would be a structural issue. They are usually in excellent locations given the need for parking in these same locations. Given it would be a top floor, one would probably have incredible views, fresher air, and less sound pollution.
You’d be creating parks out of thin air 🙂
Every big green polygon is a parking garage and the biggest one on the right is only a lot for a bunch of buses in down town Santa Monica. They could be parks…
Issues might be irrigation, exhaust from the cars parking, manic wall street traders throwing themselves from the top, etc.