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Pier I cafe at 70th street (or around that) on the water. An amazing place to hang out on an evening. Sangria is expensive, but great to relax with. It gets crowded, and check that it is open before you go!
I'm a big fan of the East Meadow it was re-done sometime in the last few years and is super nice. It is a huge open green space (with some trees for shade at the sides) and generally has the least amount of people. http://www.centralparknyc.org/visit/things-to-see/north-end/east-meadow.html