Since you are driving down, it will be easier to stick to healthy breakfast and "throughout the day snack" options. We are going to be driving down too and will have cereal for breakfast- cheerios, multi grain cheerios and then a "treat" cereal for the kids, probably Corn Pops....even that is still healthier than chocolate croissants and pastries.
We also will be packing apples, bananas (keep them separate from other food so u don't have all the other food banana flavored). We were also thinking to bring yogurts, animal crackers, frozen grapes, graham crackers, pretzels, mixed nuts, granola. My husband said "everything but the kitchen sink". lol We also are bringing whole wheat bread to make PB&J sandwiches. You can pre-make hard boiled eggs and keep them in the cooler than transfer them to your fridge; those are good for morning breakfast; along w/ peanut butter toast. Then there is instant oatmeal. There are so many possibilities. These are all on our list.
Health(ier) snacks in the parks:
Dole Whip is a lot less calories than the mickey ice cream bars
Veggie burgers, veggie wraps instead of cheese burgers
Ask for dressing on side
Mickey shaped pretzel
Our mindset is to eat like we normally do which is healthy, however it is vacation and we will eat "bad foods" too, in moderation. Good thing is you do so much walking, it's ok to eat some junk. I feel better though when I eat healthier, so the majority of the time we'll be eating all the foods I posted above.
For TS meals, the yak and yeti had a wonderful salmon, i'm sure that was 1 of the healthier options.
We were thinking of doing the character buffet breakfasts but I hate eating too much at once so that will not be good for us- we like to eat small meals frequently throughout the day. Buffets are so not for us!
Good luck, i'm sure you will be able to incorporate your healthy eating in your Disney trip.