A day and night, or just a day? I think Central Park is underrated. I'd at least walk across the lower section of it, from east to west. Take the 1/9 downtown from columbus circle (actually, I think it's just the 1 now - the red one). Get off at 14th street and walk south through the Village and toward city hall. Maybe check out the Chinatown/Little Italy area - although not a must. If you get a chance, walk across the Brooklyn Bridge. Try to find a good pizza place and grab a slice to go, or a hot dog at Papayas. Just walk around, stop in a pub or three if you can. Maybe stop by south street seaport area, because the water is another underrated part of Manhattan (Battery Park on the west side is another option). I wouldn't recommend WTC, even though most people ask about it. There's nothing but a giant hole, construction site and office towers there. If a night is planned, grab a Zagat's, do a great restaurant and drink til dawn. There's a population of people who swear they've never been above 14th street. I think they are insane, and lying, but if you do have one day in NY, you can get all you need south of 14th (and if you have a handheld GPS, bring it). I think things like Empire State Building and Times Square are unnecessary. Wall Street doesn't offer much. Most of midtown is just office buildings. Rockefeller Center is nothing special. Plus you'll just get in my way when I'm trying to get home.