I've lived in NYC my whole life and never appreciated how pretty the beach at Coney Island is. I'm sure if you go during a warm weekend day, it's a mob scene, but it's picture perfect otherwise. The wide beach will make you forget all about the city.
Family friendly beach, with fun surf and beautiful dunes (though the dunes have been significantly eroded in recent years). Pro tip - go at low tide and enjoy the dramatic changes that the beach presents to you.