G.B.
You could find a bunch of foods they like then make a weekly menu with lunch on it. Then when you go to the store your shopping list would have those items on it. This will help you feel like you don't have anything to prepare for them. Stocking up on the staples like soups or other items you like would also be time consuming and help you in the long run to be better prepared. I stink at this and have learned....we keep tons of Spaghetti O's and cans of Mini Ravioli's in the pantry.
I also did simple stuff like mac and cheese, hot dogs (we use Hebrew National, they don't make me sick for days), grilled cheese and chicken noodle soup, fish sticks and a pretty veggie to contrast the beige sticks.
PB&J is not a bad food, it's full of protein.
Here's one of my main resources for all kinds of things.
http://familyfun.go.com/magazine/
Here's the link to recipes
http://familyfun.go.com/recipes/
And here's a link to lunch ideas for kids
http://familyfun.go.com/recipes/lunch-ideas/