You might change your mind once you get there and are enjoying everything. You might want to enjoy a couple of rides one more time, you might want to have another butter beer, or pumpkin juice, you might want to find the little secret things around the parks and enjoy them. Such as for example, at Borgin and Burkes, you can see the vanishing cabinet and hear a bird in there, but what a lot of people don't know is that if you put your hand on it, you can feel something moving in there. It's little things like that which are hidden at the parks as well which a lot of people don't think about. A lot of these things also can be found online, but at the park itself, it's not advertised, or common knowledge. So, it's things like that which are fun to find!
Also, you might want to focus on photography, if that is something you're interested in, if you're busy the other days trying to take in the rides and shops. Or perhaps, go to the shows that are there, or experience other restaurants which you have not tried yet in the park. Or just walk around and take in what you haven't seen.
It doesn't have to be a day loaded with, let's do this, now let's do this, now let's do that, because when you first get there, you'll constantly trying to get from one place to another to see
everything, but the beauty of that third day is that it can be a day which you can take your time, enjoy the scenery, and catch what you didn't see before. Or as
@Seeker615 said, it isn't all just rollercoasters, most of the rides are simulators, or another form of a ride, or attraction. Islands of Adventure/Universal, not just WWoHP, might surprise you.