Having the experience of owning and running a coral reef aquarium (and frequenting this particular shop about once a week) - I was sad to hear it closed it's doors. Their reef display tank was always amazing. These pictures were taken from PetCo at Union Square (I also used to go there for my saltwater needs).
My second-favorite outdoor-place in Manhattan. The abandoned 1.6 km rail tracks running above Chelsea down to the Meatpacking district were turned into an above-ground park. Perfectly nice place for sitting in the sun, reading, relaxing, having lunch in some green spot in the city. Nice views at the Hudson River, New Jersey, the Statue of Liberty (little small from there but still visible) and the architecture around Chelsea.
A place for when you only have a day and your soul needs the smell of an ocean breeze and to feel sand between your toes. You can watch all the craziness of Coney Island from across the water.