If there is one thing I've learned from my previous trips to NYC, it definitely is that you shouldn't go there unprepared. A pair of good walking shoes, some spare cash, and a little knowledge about the whereabouts are highly recommended. The city is so big, it can take ages to get from one site to the next, so it's definitely handy to have a map with points of interests ready. This way, you'll avoid running around clueless and missing many cool spots on the way.