I spent a while with:

http://www.farecompare.com/search/flyertalk.html

Put NYC in Departure Cities, check North America, and sort on Total.

I'd see lots of possibilities, but when I chased them down by clicking on them there always was a higher fare for the immediate few days. After several days or so, the lower fare would appear.

If you are still interesting in pursuing this, maybe you could change your idea to planning a week or so ahead?

Looks like those airlines just aren't into spontaneity!

Romelle