If its not that cold out I'd say turn up the Ipod, and take a walk. Start from Lower Manhattan area around Wall Street, Battery Park City, or Hudson River Park and just walk straight ahead.
If you take all Broadway (or plot a route on Google maps) you can see Chinatown, SoHo, Union Square, Chelsea, Harold Square, Koreatown, Times Square, Columbus Circle, Lincoln Center, and Central Park in about 2 - 3 hours depending on how fast you walk and foot traffic.