Unfortunately I don't think that's enough time to leave the airport. It's about an hour from JFK to Manhattan, so leaving enough time to responsibly make sure you're back at the airport for an international flight would leave very, very little time to do anything.

I guess you could go somewhere closer to the airport, like Rockaway Beach, but this time of year, when it's cold and dark by 5 p.m., that doesn't sound very appealing.