-Nothing fried
-no sodas (not even diet)
-no snack machines unless they are filled with healthy snacks
-no pizza's (and the reason why I say that is because kids generally get it on their own quite a bit, and the cafe always serves it very greasy, so you might as well just eliminate it)
-go with fruits and veggies, and if you can, try to have a very organic meals. I've heard of several schools going organic, so you might want to do some research there to see how they managed this, and what their budget was like.
-have make-your-own-sandwich bar, with several different breads instead of white. Could have a pita, wraps, etc.
-can have posters of foods and their benefits up on the wall. Example: milk, calcium etc.....it just be an interesting way to pass the time in the cafe.
Hope these are useful!