Whichever of the four “theme parks” you have a reservation for must be the first “theme park” you enter that day. But you can use one of your “plus” days at any of the non-theme park attractions (including water parks and mini golf) at any time. So if you have a reservation for MK, if you go to BB in the morning, you will need to go to MK in the afternoon before you can go to any other theme park. Or if you decide you want to go to, say, EP instead, you need to change your reservation to that park. So it is helpful to know in advance which theme park you want to go to first, not necessarily which you will go to in the morning.