I had alot of grilled chicken and baked chicken in WDW and thought they were overall terrific. These places included: 50's Prime Time Cafe (DHS) & Sci Fi Dine in (DHS). The baked chicken at Boma was terrific. I didn't care for the baked chicken at the Biergarten. I also really liked the roasted turkey at LTT character dinner. Turkey sandwiches were very good at Captain Cooks (Poly) and Main St. Bakery. Captain Cooks also has a steamed veggie side dish.
The trick is to find out what "extras" they put on the menu items. Turkey Sandwich tends to not be as healthy if it comes on a croissant and/or with tons of cheese and mayo. Find out if any chicken or turkey dishes come with any sauces you might not like. I asked for many things plain, without cheese, or with sauce on the side. All restaurants were very good at accomodating this.
I was also able to substitute fries for fruit, sliced tomatoes, or cooked veggies.
Things I didn't try, but noticed they had alot of was seafood, especially salmon. Again, find out if they are served in any sauces in case you are concerned about that. Sauces can sometimes ruin a once-healthy meal!
I've heard that Le Cellier serves fairly lean steaks.
You should also be able to find lowfat, nonfat, and no sugar added frozen yogurts and ice cream.