Well, if you get stir-crazy, you can ride the Airtrain all you like but then, you'd have to go through security again which can take a LONG TIME. In the summer, there can be delays in flights in and out of JFK so you might arrrive at JFK later than you think.
2 1/2 hours really isn't that bad. Here is a list of shops and restaurants for each terminal. Find out which terminal you'll be departing for Jamaica and then find a nice bar or restaurant. Or go shopping. I hope this helps.
http://www.panynj.gov/aviation/jfkshopsframe.htm