If I stop by wendy's, I'll usually have either their grilled chicken sandwich (no mayo), or one of their grilled chicken salads with fat-free dressing. I might also throw in a plain baked potato and/or side salad (if getting the sandwich), depending on how hungry I am.
Edit: Also forgot to mention their chili, which I believe is surprisingly healthy as well.
Edit (again): Just remember that if you're going for healthy, stay far, FAR away from any of the breaded and fried chicken sandwiches. They often times will have more fat (and trans fat) than the hamburgers.