I went to NYC in March last year for 5 nights.
Plenty to do, we only left Manhattan to go to Liberty Island.
The weather isn't that bad. Was a bit cooler than the UK, very mixed conditions - overcast one day, bright sun the next. Got drenched on the way to a show in Broadway one evening.
My recommendations:
-Get a hotel somewhere fairly central in Manhattan. We stayed at the Affinia Manhattan (kinda opposite MSG) and walked nearly everywhere except to and from the airport and getting a bus to Battery Park.
-Macys is fabulous (god I'm such a girl). I assume they'll have massive clearance sales like they did when we went, plus you can get an international visitor card for another 11% off everything. Of the casual clothes I currently wear regularly, I bought about half of them in there.
-On a cold day stop off in Central Park Zoo, they've got a tropical area that warms you up nicely

-Do the Rockerfeller/ESB late at night. We went during the day and they were really busy, you felt like cattle at times.
-Eateries generally seem to fall into two categories: Pretentious and somewhat overpriced, or common and good value. A lunch for two can easily run to $100 in a restaurant, or a more reasonable $15-20 in a small chain outlet. We never really found anything in between, but I'm sure they do exist.
-Cheaper seats in MSG are better than I was expecting.
This probably sounds a bit weird/sad, but one of the great things I've found having been to NY is that when I'm watching films etc, I can recognise places. It actually makes a real difference. The street naming convention also makes perfect sense to me now.