We just added the Plaza Restaurant at MK to our meal itinerary for next week and I noticed that they have a vegetable sandwich that sounds very good. I haven't eaten there before so I don't know how light it is but it sounds yummy. It's a sit down restaurant but I THINK it is a bit less crowded and informal than the other restaurants. I made a PS but I don't think this is a restaurant that many people make PSs for. Here's the description:
Fresh Vegetable Sandwich - fresh spinach, hummus, grilled portobello mushrooms and roasted red peppers, rolled in a tomato-basil wrap served with sweet potato chips $8.99.
I'm not a big fan of most of the restaurants at MGM but you can get some fresh fruit, pickles, etc at the area off to the left as you head toward ToT (can't remember what it's called).
At Epcot I really like the Tangerine Cafe in Morocca. You can have a nice vegatarian meal there with an assortment of salads, hummus, and Moroccan bread.
We usually eat at the Rainforest Cafe at AK, so I'm not familiar with the counter service restaurants there.