As previous poster stated, you don't have to stay on property to leave and come back to same park on the same day.
We like to get to park early, take a break mid afternoon and then go back to park either for dinner and night time entertainment or after dinner for parade/fireworks.
This is especially a good idea if you are going during the hotter summer months. Spend the hottest part of the day at the pool or relaxing and enjoy the parks when they are the least crowded -- when they first open and then later in the evening after all those people who stayed in the afternoon are so hot, tired and miserable that they've gone back to their rooms to collapse.