It depends on what your interests are. Museums, restaurants, shows, etc?
Some suggestions:
1. Metropolitan Museum of Art
http://www.metmuseum.org/
2. Guggenheim Museum
http://www.guggenheim.org/
3. St. Patrick's Cathedral
http://www.ny-archdiocese.org/pastoral/cathedral_about.html
4. Circle Line cruise for photo ops of Manhattan from the East River and Hudson River
http://www.circleline42.com/index.ihtml
5. Ground zero
6. Off broadway show Stomp
http://www.stomponline.com/
7. Sushi pizza at the Mercer Kitchen in Soho
http://www.allinfo.net/jg/mercer.htm
8. Peking duck, Crispy Beef, and Steamed Dumplings at the Peking Duck House in Chinatown
http://www.chinatownweb.com/PekingDuck/
9. walk around Greenwich Village
http://www.nycgv.com/
10. Hayden Planetarium
http://www.amnh.org/rose/haydenplanetarium.html
11. If you have the funds, dinner at Le Bernadin
http://www.le-bernardin.com/
12. Empire State Building Observatory for photo ops from the highest point in the City
http://www.esbnyc.com/index2.cfm?CFID=7521480&CFTOKEN=21918614
13. Staten Island Ferry for photo ops of the Statue of Liberty (best bargain in town and no lines necessary like those for the actual ferry to the Statue itself, especially since you can no longer go up into the Statue)
http://www.siferry.com/
14. If you are into photography, go to the best photo store anywhere, B&H Photo, on 33rd and Ninth.
http://www.bhphotovideo.com/bnh4/controller/home
15. Romantic dining - One If By Land... (order the Beef Wellington)
http://www.cityguidemagazine.com/nycreviews/oneifbyland.html
SUSHI: It is highly unlikely you can get in, but see if Nobu has any last minute cancellations or you can try to time it right to get into Nobu Next Door which does not take reservations. You will not have an equivalent level of sushi/Japanese anywhere else in the city except for the new restaurant in the Time Warner building at Columbus Circle (for around $500 per person).
http://www.myriadrestaurantgroup.com/Restaurants/nobu/Nobu Main.htm
Shopping tip: watch out for tourist trap stores with "lost our lease" or "going out of business" signs along Fifth Avenue. Madison Avenue has the upscale boutiques.