I was going to suggest seeing if there was a way to get to North Brother Island, but apparently it's quite difficult (see here. You could go guerrilla, but I suspect that's even harder.
I posted some other links with photographs etc a couple of posts down this page.
Also, it's probably well worth checking out the following link which I recently came across while looking into another matter. Like so many things you trip over on the net, it was kind of irrelevant (although interesting) in regards to my search then, but has now found it's purpose: New York Secrets. I love it when that happens.
- It's surrounded by less-than-scenic / less-than-'upper shelf' areas (e.g., lots of industrial facilities; the Rikers Island detention complex; residential neighborhoods not known for affluence)
- North Brother Island has a long history of serving as a place for isolating diseased people (e.g., Typhoid Mary), and it was the shore nearest the General Slocum disaster. The place might therefore induce 'the creeps' in some folks.
- Development would require overturning its status as a wildlife sanctuary - a big project in and of itself.
- Any development project would entail huge expenses for demolishing the many decrepit buildings, etc., still decaying in place there.
The video comment about the disincentive resulting from being under the flight-path for La Guardia Airport struck me as a very odd factor. Surely NYC is awash with every conceivable noise at all times?