Another one of my favorite writing exercises is to sit with a pen and piece of paper and follow my stream of consciousness. This usually helps me put down a great number of thoughts, even if they aren't connected to one main idea. I can then reread what I've written and look for a topic that I believe I can weave into a story.
Another one of my favorite places in NYC is Grand Central Station. What I love about Grand Central Station is the immense open space inclosed by the spectacular dome ceiling. It is a place crowded with people either trying to reach a destination or arriving home. Grand Central Station is one of the finest of New York City's historical sites and a place that has the potential for generating many stories.