Just walk into any seminar you find interesting:
http://math.columbia.edu/calendar/main/one/next_week.html
http://www.cims.nyu.edu/events/
http://www.math.poly.edu/news/seminars.phtml
http://math.gc.cuny.edu/seminars/bulletin04_13.pdf
No need to contact anyone in advance. If you need directions on how to get to any of these places, just ask and I'll post them.
ADDED: It's also very easy to do day trips by train to neighboring universities:
http://www.math.sunysb.edu/html/seminars.shtml
http://www.math.rutgers.edu/seminars/calendar.php
http://www.math.princeton.edu/seminars/
https://www.math.upenn.edu/cgi-bin/calendar/view.pl?item=NextWeek
MORE: If you want to give a talk instead of listening to one, I suggest you find a seminar where the speaker isn't going to make it from Europe and where the organizers know either you or your work. You could then contact them and offer to be a substitute.