My favorite beach in NY is practically right in my backyard-Coney Island! I have gone here all my life and for a while about 20 years ago, I would say it wasn't a very safe place to go anymore. However, it has undergone a revival in recent years. The area is very nice and safe now and the beach is cleaner than ever.
When you get sick of lying on the sand, only steps away is the New York Aquarium where you can see every type of fish and even sea lion shows. Take in a Cyclone baseball game at Keyspan Park. Take the kids on some kiddie rides at Deno's park or Astroland Park, all right on the boardwalk. Right down the street is Nathan's Famous, which has the BEST hot dogs.
I really don't know any other beach that has so much to do. It's also very easy to get to via either bus or subway. Coney Island is the best!
Here is a website with info for Coney Island:
http://www.coneyisland.com/
A link to the NY Aquarium:
http://www.nyaquarium.com/
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