Everyone goes to the High Line but honestly the elevated walkway at Domino Park in Williamsburg has a better view of the Manhattan skyline and it is never as crowded. There is a full basketball court, a dog run, and a really good taco stand at the south end. The park used to be a sugar refinery and they kept some of the old industrial structures which gives it a cool vibe. Best time to go is around 6pm when the sun hits the buildings across the river. Free entry obviously.