In park order, here are some choices:
MK-
Not necessarily a healthy choice, but I love the waffles at Sleepy Hollow.
Columbia Harbor House (when it is open) has veg chili and the lighthouse sandwich - hummus and broccoli slaw.
Casey's for veggie hot dogs.
Pecos Bill changes the offerings here and there but they have veg fajitas or tacos or something along those lines.
Epcot -
Brie sandwich at Les Halles plus a bunch of pastry choices, cheeses and baguette
Empanadas and plantains at Cantina de San Angel
There are always veg options at Sunshine Seasons
Vegetarian menu at San Angel Inn
Veg sushi and tempura at Tokyo Dining
Hummus fries and crispy cauliflower at Spice Road
Pizzas! at Via Napoli
Animal Kingdom -
Veg sausage at Flame Tree
Tofu bowls at Satuli Canteen
Impossible sliders, kids falafel, breads and dips at Nomad Lounge
Lots of choices at Yak & Yeti!
Hollywood Studios -
Kefta at Docking Bay 7
Plant based totchos at Woody's lunch box
Curry rice bowl at ABC Commissary
Also - dole whips at Poly and Adventureland in MK, gelato in Italy at Epcot and ice cream sundaes at Beaches n Cream.