I worked in a preschool and there were all kinds of things people send. Here are some ideas I've seen and used myself.
PB&J sandwich cut into small pieces (like cut into 1/4ths and then cut the 1/4ths into 1/4th)
rolled up deli meat
cheese cubes/string cheese
crackers
goldfish
raisens
banana
mac & cheese in a thermos (it stays warm, but isn't HOT by lunchtime)
turkey pepperonis
cut up grapes
sectioned oranges
fruit cups
pudding
yougurt drinks/cups/tubes (if you freeze a yogurt tube, it will be cold at lunch)
cold pizza (not me, but I've seen it!)
cut up hot dogs/vienna sausage
Granola bar
There really are tons of things. Think about what you fix at home and as long as you aren't heating it, you should be fine. You can also heat stuff up and put it in a thermos. You can even ask the teachers if it's still warm at school so you'll know what works and what doesn't. Not for my little one, but for my older ones I've put cut up hot dogs, pizza, etc. in their thermos so it will be warm at school.