It's not really a "park", as such: it's a little plaza surrounding
the 72nd Street stop on the 1 train in NYC.
The reason it looks so f'd-up in the movie is because that area used to be a slum.
Then scum lower than the area's former inhabitants came in and made gourmet cookie stores .
Thus, the Upper West Side is now what you see in "Seinfeld" or some "hip" NYC movie.
Around this plaza is lots of shrubbery.
A favorite pastime is throwing individual "Reese's Pieces" into the bushes and then watch them
come alive as God-knows-how many rats scurry around for the nutty goodies.
Yes, I'm quite familiar with that "park" growing up,
but only because it's where Riverside Drive begins, and it's a pretty busy area.
Plus, you get off on that subway stop go to the Museum of Natural History a couple of blocks over
on Central Park West. Well actually, you should get off
at the 79th Street stop, but tomato/tomahto, y'know?

The scene where they're all huddled together on the park benches was definitely on 71st and Broadway. If you look it up on google earth you can still recognize it, although the entire area has changed a lot.

At the beginning of the film the caption said it was Sherman Square.