Happy Equinox! And welcome to the NYC Naturalists community. There are lots of urban naturalists here in NYC. Whether you’re a generalist, a specialist, or somewhere else along that continuum, there’s lots to see.
There are so many great responses above already. I’ll share some of my favorite ways to iNat NYC.
Considering plants alone, there are over 3,000 species documented in NYC by over 18,500 observers.
“Weeds” are a rich source for observations. Find an unkempt hell strip - or broken up driveway - and see how many different plants you can identify.

With improvements in air quality over the past few decades, lichens can now be found nearly everywhere in NYC. Many of our street trees have some. Some even have mosses growing on their trunks.

Cemeteries are also good places to look for lichens. Green-Wood Cemetery in Brooklyn is known for this, with nearly 500 species of fungi documented so far.

NYC is on the Atlantic Flyway, and thus is a hotspot for birders, especially during Spring and Fall migrations. Except for the occasional escaped parakeet or poultry, nearly every bird you see is wild.

From my photos above, you can see I’m more on the generalist end of that continuum. My specialization is insects. I’m especially keen on observing insect-plant interactions. Galls and leaf mines can be found even when plants are not flowering. Things really heat up when things start to bloom.

As @klodonnell mentioned above, City Nature Challenge is coming up. There will be meet-ups all over NYC for that. It’s a chance to get to know some fellow iNatters and see other parts of the city with a welcoming group. Events will be listed on the main project’s journal.
Hope to see you out there!
Yours in Nature,
@xris