The DBf and I have to eat healthfully when we go, as we're locals and on diets right now.
At some of the counter service restaurants, there are toppings bars, which we use to load up on veggies for our sandwiches and wraps. It makes the meals more filling and seem more substantial when it's a chicken wrap, grilled chicken sandwich, etc.
Some of the counter service restaurants offer baggies of carrots or fruit instead of fries or chips. Definitely ask and take advantage of that wherever you go! They'll do that even at the water parks.
As for sit down restaurants, if you know where you're going in advance, check out the menus online. If you can't do that, I believe most of the restaurants have healthy choices and can make substitutions willingly.
There is at least one fruit stand in each park, and we take advantage of it. The prices aren't *too* bad for what you get. On the ice cream stands, they have fruit-ice pops that I believe should be under 100 calories compared to what they sell in grocery stores.
We also usually carry some form of snack with us just in case - always water, somtimes dried fruit and nuts, a granola bar, things that travel well.
And if we really want to treat ourselves, we'll order one meal/item that's bad, but not TOO bad, and split it along with the healthy meal that we choose.
Good luck!