Mary, those forums Abear pointed out have lots of great ideas.
One of my kids' favorite things to bring for lunch is bagel and cream cheese. I slice the bagels, bag and freeze. The night before, I open up the bagels, spread the cream cheese, then put in the sandwich bag in the fridge. It's completely defrosted by lunch & stays cool with one of those little icy packs.
They also like tuna on burger rolls (especially potato rolls!) That's what they're taking today. My 2 year old had it for breakfast, since he wanted what they were having and still has no concept of "wait until lunchtime."

I can't handle tuna at 6:30 AM

but he ate it. At least it was a nutritious, if strange, meal!