Central Park is huge, like 844 acres. There are infinite places that would be great for picnicking but in case you’re unsure here are a few of my favorite spots.
Sheep's Meadow, which on a summer day looks like a beach with thousands of people sunbathing having a blast. It’s on the lower half of the park so easy for anyone living downtown to get to. Its perimeter is lined with trees (where you’ll find me) providing some nice shade on a hot sunny day. There are also 2-3 public bathrooms that are a 5 minute walk, key if you plan on consuming booze during your picnic. There are also vendors like the ‘Mojito Man’ that will be selling highly potent drinks, so bring cash.
The Great Lawn, which is the next large field to the north, is also great for hanging out. It is lined with baseball fields so you might be able to watch some local games while socializing. It also has a bathroom to its southwest and northeast.
I personally like to hang out behind the Met on the far eastern edge of the great lawn. Specifically by Alexander Hamilton. You get to watch all the bikers, runners, rollerbladers and whatever else zip past in front of you on East Drive.
And finally, for a quieter more ‘local’ vibe head to the East Meadow which is just off 5th Avenue above 96th Street. Due to its northern location it tends to be a bit less populated but equally grassy and fun. The field is a bit of a bowl, making for great people watching if you sit along its higher edge.