I would absolutely NOT plan on anything other than getting from one airport to the other. You won't have time to do anything else. Frankly, for your sanity's sake, I'd try to re-arrange your flights so that you're coming and going from the same airport. You have the island of Manhattan between your two airports, a virtual Charybdis of traffic flow that will defy any and all attempts to cross it in a timely fashion. One sunday afternoon I was trying to leave Manhattan to return to DC in a car. It took TWO HOURS to get from Times Square INTO the Lincoln Tunnel (a grand total of 8 blocks). YES, it can be that bad.