New York City Marble Cemetery
I recently posted photos from my visit to the New York Marble Cemetery, and mentioned that there is an un-affiliated New York City Marble Cemetery a few blocks away. The NYCMC is slightly more accessible than the hidden NYMC, and although the gate is normally locked, you can see it all from the sidewalk.
It's open once a month from May-October and it's a beautiful space to spend the afternoon. When I was there it was pretty crowded and in addition to people, the cemetery was filled with pigeons—in the trees, on the wires and on the headstones.
The NYCMC has headstones like a traditional cemetery in addition to underground vaults designated with flat marble markers. It also has a fantastic iron fence and gate, with a wonderful tombstone-shaped sign and arching letters. It's not as secluded as the New York Marble Cemetery, but it's a great place to spend the afternoon and have a picnic amongst hundreds of circa-1800s graves.