I would take them to the supermarket and get them to pick their own? That's what I do with my DD and she is great with her lunches..
I found that if I try to give her anything that the other kids don't have then she is a little more reluctant to take it.. It's all about fitting in with the other kids!
A usual lunch for mine is a Vegemite sandwich, muesli bar, a bit of fruit, and one piece of junk (like and LCM or Le Snack etc) and she gets a poppa.. I find this is perfect.. If I try to change anything (I once tried to substitute to vege sandwich for a ham and cheese wrap) that's when I get issues!