Again Ive done this the other way round like Disneymad. It was very easy, spend a few nights in NY and then fly down to Orlando. There are certain advantages in doing this if you time the flights to Orlando to get in before the jumbos coming in from the UK, ie not queues at the car rental checkins and you get to the hotel early and stand a better chance of getting a room change if you want one.
As far as a day trip is concerned I wouldnt have thought it possible or worth it. The journey to and from JFK recently can take well over an hour with the major road works going on, plus all the checkin time with the security checks.
The flight when we did it only cost $60 one way, we flew into NY and out of Orlando with BA.
Boston is another great place to do the same thing with, nice and easy to spend 3 days there and then 11 days in Orlando.