Short version of my advice:
- Arrive first thing in the morning. Way before official opening.
- Stay in the parks. Less mess, less drama. I mean, can't worry about not getting back in, or not that way.
- Have a nice meal and/or enjoy shows during mid-day when you want to relax.
The long version / explanation:
The park can even reach capacity before the official opening depending on how busy it can be. Know this is possible as it happened during the Christmas holiday season. During most of those busy days, they didn't necessarily close before official opening, but closed often within minutes, or roughly only an hour of being open. So, it's essential to arrive early, because whether it gets to capacity, or not, as the day goes on (depending on how busy it is) the wait times add up due to the limited capacity of the rides.
Staying in the parks guarantees you won't have to worry whether you may, or may not come back. However, as others stated, it is possible to come back in. However, the key word is possible. That being said, I personally if I really felt I needed, or planned to stay as late as possible. I would just stay in the park. While the leaving part may be easy, you'll have to spend time and effort coming back into the park (especially if it does reach capacity).
You can find plenty of places to eat and relax. You have U-Rest areas in order to sit down, take off your mask, and relax. You also have plenty of table service restaurants in which you can take your time to relax, eat, and enjoy the air conditioning. You can also save Horror Make Up Show and the Bourne show as a way to relax in some air conditioning as well in the mid-day. While Sinbad isn't open, it's one of the U-Rest areas in IoA and for fans of theme park history, or used to be a fan of the show, it's a nice place to relax and really admire the stage/set as it's still intact.