Mix it up a little bit. Don't always pack the same thing. Try different fruits and vegetables. Slicing them up makes them more fun to eat than eating them whole (ex. apples, oranges, strawberries, carrots).
A trick I learned recently is to freeze yogurt then put it in your pack. It works as an ice pack and frozen yogurt doubles as a treat, it's just like fruit flavored ice cream. It's a new personal favorite because I'm not a huge fan of normal yogurt texture.
Small portions of several different things. This way you can throw in something slightly less healthy as a treat but it's still okay because it's only a small portion.
I also recently discovered those crystal lite flavoring packets. They're only 10 calories and are perfect for putting in water bottles. Much less sugar than juice or pop, though if your kid is still in school and can get milk for lunch try to convince them to do that.