I usually pick a serving of fruit, vegetable, grain and protein to ensure a well balanced lunch. My kids eat at school maybe once a month since I think the school lunches are processed foods and junk.
Both of my kids are content with a sandwich (either meat and cheese or natural pb on whole wheat) so that takes care of grain and protein. DS drinks milk at school (provided) so dairy is taken care of but for DD I usually put in a yogurt. Then they each have their favorite fruits and veggies and about a 4oz serving is right amount.
I also have thermos for each of them, so left over soup, lasagna, etc can be heated up for that. I have done the bento box for them as well, but it takes more thinking and planning to do that than I have time for.