Hey,
JFK is not only huge, but isn't really close enough to anything to risk travelling into the city to sightsee. It would take over 2 hours just to travel back and forth and you'd have to leave your luggage at the airport. Not worth it in my opinion, but you could ask when you get there.
Plus, you are technically supposed to be back at the airport 1-2 hours before your flight, so that cuts your real "leisure" time down to just 4 hours or so, max.
JFK layover