Roosevelt Island is a gem that goes overlooked by most NYC visitors. The island has over 147 acres of parks and promenades, lots of riverfront paths, gardens and incredible views of the city. Take a taxi, the F train, or take in the views on the air tram across the East River. The Tram costs $2 each way, MetroCards only.
The High Line is a recent addition to NYC parks. From the remnants of the abandoned elevated train line that serviced factories in the Meatpacking District, NYC built an elevated park and trail. The High Line stretches about a mile and a half from the Meatpacking District, up through Chelsea to 20th st. You’ll see people strolling and enjoying the views, and if the weather’s still nice, even sunbathing. Since it passes directly over the Chelsea Market, be sure to stop by there and pick up lunch; many restaurants in the market offer High Line picnic lunches to go.