Assuming this is the first visit to NYC you must do some of the most common but worth to do activities such as:
1) Museum of Natural history
2) Emipre State Building
I also suggest a walk through the streets of SOHO for some shopping or maybe just window shopping. While here you can stop for lunch or dinner either at Il Barolo or at I tre Merli on the same avenue West Broadway and same owner. Down the avenue there is also Cipriani Downtown location If you are in the mood for a simple Panini sandwich you can look for Café Borgia off of West Broadway on Spring Street.
Speaking of Cipriani you should book one night the Rainbow Grill (same link above) at Rockfeller Center where you can enjoy a beautiful view on Manhattan.
Manhattan has to offer great views also at the Hotel Marriott Grand Marquis at Times Square here there is “The View” Bar and restaurant you can go here for a drink, the bar rotates around the building and you get to enjoy a 360 degrees view of New York.
Also not to be missed if the weather allows, at 59th street and 5th ave the Peninsula hotel offers a rooftop garden bar which is elegant and offers a different view in fact you can see the stylish fifth avenue from the top, nice atmosphere worth a drink .
One night you can book the World Yacht dinner cruise http://www.worldyacht.com/ which certainly is a different way of dining and enjoying the view of Manhattan from the water.
Last week we took some friends from Italy to a Brazilian restaurant http://churrascariaplataforma.com/ it was great nice atmosphere excellent food but you must love meat!
Speaking of meat a steak house is a must and I can suggest Gallagher’s http://www.gallaghersnysteakhouse.com/index.asp
To end the night if you have energy and like to walk you can cross the Brooklyn Bridge on foot, something different and absolutely unique views here some info http://www.nyctourist.com/bridge2.htm Actually this can be done during the also but I only walked over the bridge at night and New York at night has a unique charm.