This restaurant seems to the best job catering to vegetarians so I'd consider that one. Look at the menu, but also give them a call and speak to a chef. Generally you pay less for lunch than dinner(Boma doesn't do lunch) and its better to eat the heavier meal mid day and make dinner lighter. Good luck with your effort to eat healthier and lose weight.
Keep in mind drinking lots of water (especially at Disney) and eating lots of fresh fruit are too of the best things you can do as healthier options.