As a college student in Manhattan, the first landmarks I learned to find were campus and the favorite spots for students and other Manhattanites looking for an evening out. This cache is placed near my favorite weekend people watching spot, close to campus and many local hang outs that ensure a good supply of people to observe. In the evenings, colored Christmas lights or a mini-outdoor movie theater sometimes appear nearby to add to the atmosphere and make this area a Manhattan must see.
Beware of high muggle concentrations (but best people watching) if you chose to hunt for this one on weekend nights while the pedestrian population in the area is at its peak. During the rest of the week, traffic in the area should be fairly low and sneakiness required will dramatically decrease.
Due to the potential for muggles in the area, stealth is a must! You are looking for a magnetic key holder, BYOP.