Anything. Meat can sit out for a whole day before being eaten. I do it all the time.
Just eat a protein source (meat/fish) and bring fruits/vegetables for lunch. Maybe some nuts too.
Also if you're really concerned about it, get an insulated lunchbox and keep a re-usable ice pack in it. Target has some really cute ones.
I usually pack leftovers from dinner the night before, and just eat it cold. Not the best, but it does the job.
Other suggestions for protein in your lunch, if you'd rather not eat cold leftovers, include:
Salad with boiled eggs and tuna
Sandwiches: make a big batch of egg, tuna, or chicken salad on the weekend, and spread it on whole-wheat pita or a wrap rather than bread. Or eat it plain. Or stuff it inside a hollowed-out tomato.
Yogurt. Plain is good, that way you can control the sweetness yourself by adding just a teaspoon of jam or honey. Low-fat or fat-free yogurt tends to be really high in sugar, as does flavored yogurt.
Cottage cheese
String cheese
Pepperoni or Canadian bacon slices (in moderation; they tend to be highly processed and filled with salt)
Or just slice up whatever meat you had at dinner, throw it over a big bowl of greens, sprinkle some crumbled feta or sunflower seeds on top, and voila! A salad.
Hope that helps.
