Hyunkie, you probably know about more places in the city than I do...even though I've lived outside of Manhattan for the majority of my life. There is so much to see everywhere in the city...usually I'm driving when I'm in there and can't take all the photos I see with my eyes.
There are some great walking tours, many are free... You can read about them in the NY Times and these days they're probably online.. Historic walking tours..whether they're in the financial district or parts of Brooklyn or Green-Wood Cemetery... That might be an interesting thing to try with your camera in hand.
Please be sure to post some of your photos in the image threads or genre forums, Hyunkie!