Another PB&J variation is PB and Banana - put the peanut butter on both slices of bread so the banana slices don't slide around. It's surprisingly good; salty and sweet.
If you have a microwave, a good ethnic grocery will have microwaveable non-frozen Indian food. Usually some sort of Dal or other vegatarian offering that is stable at room temperature.
I agree with the idea of getting those little blue cooler things to stick in your lunchbox. We use it for my kids and they work well. To be honest, if my wife hadn't got them I wouldn't have bothered. I used to take tuna fish or meat sandwiches to school in a paper bag and never suffered any ill effects. Eating cafeteria food was always more of a gamble :-)
Another option is tunafish and crackers or some of the prepared tunafish meals you can buy off the shelf in the tuna can section of your grocery.
