Turn it off.

Turn it off if you aren't using it. Our parents were right we're not trying to cool the outdoors so keep the doors shut. But times change and more than likely the kids (if they are even home) will be inside and plugged into something. But somethings don't change like a light bulb that isn't on doesn't use any power. Also if that light bulb is of the CFL variety rather than incandescent it doesn't use as much energy when it is on. Another thing about CFL bulbs is they don't generate nearly as much heat for the A/C system to overcome.