We have used QSDP on both of our WDW trips and find it hard to beat for value. When I estimate OOP vs. QSDP, I really do think we come out ahead by buying the plan. We use many of our snack credits on water in the parks, which would add up quickly. We also believe, with some careful choices, the food at WDW is healthy. Of course, you will have lots of desserts to choose from, which isn't the way we normally eat.
At MK, we like Columbia Harbor House, Cosmic Rays, or Pinocchio Haus. All of them have some variety and include healthier choices. Healthy or not, you should also try a Dole whip

.
DHS is a more difficult proposition; we ate at the Toy Story pizza place this year. It was pretty good, but I'm not sure you'd call it "healthy". Starring Rolls probably offers better choices than pizza. In 2009 we tried Studio Catering and were not impressed, even though the menu does have a variety of choices.
Animal Kingdom's Pizzafari was good when we ate there in 2009. It's probably the same pizza as the other WDW CS restaurants, but it seemed to be tastier to us, and the salads were good. This year we ate at Flame Tree; the turkey sandwich was very good.
Epcot is the best park for CS/QSDP. Sunshine Seasons has a very good menu - try the salmon and salad. In World Showcase, we enjoy Tangerine Cafe in Morocco and the Kringla Og Cafe in Norway (great sandwiches!).
Alas, the pastries in France probably negate our attempts to eat responsibly

. At least you can walk most of the calories off in the parks.
Don't forget Earl of Sandwich and Wolfgang Puck Express in Downtown Disney. These are the best CS restaurants in WDW in my opinion, and you have healthy choices at both places.