Menupages is great!
Make the NYC forum on trip advisor your friend.
As far as restaurant.com, have a look at Havana Central on 46th St. Right now the certs are 80% off.
If in Little Italy, Soho check out L'asso for a happy hour specials with two slices of margherita pizza and a beer for $6.00.
Lots of great places to eat in Chinatown, be sure to check out the Chinatown Ice Cream Factory.
Both the Metropolitan Museum of Art and Am Museum of Natural History are suggested donation only.
Walk through Central Park and Bryant Park (both have websites/see what is going on while you are there)
Take the subway to Brooklyn. The Brooklyn Botanic Garden is free admission on Sat am from 10-12.
Walk the Brooklyn Bridge back into Manhattan.
The Staten Island Ferry is free and a great way to take a boat ride past the Statue of Liberty.
www.hopstop.com is very helpful in figuring out walking, subway and bus directions.
There are great hotel deals for NYC, see
www.travelzoo.com.