As long as you get your hand stamped and have the ticket that got you into a particular park, you can go out and back in to that one park as many times during the day as you want if you do not have a "park hopper" ticket. It does not matter if you are staying on property or not.
Also, if you are driving the receipt you get when going into the parking lot will allow you free return to any of the park parking lots on that day with no additional charge.