We bought a lunchbox from Personalization mall.com with our daughter's name on it to assure that her teachers always know which lunch is hers. At her preschool, most of the lunchboxes go in the fridge rather than in the kids' cubbies and I want to make sure the teacher has no question.
As for what to put in it, I send peanut butter a lot, but there are no peanut allergies in her small school. If they ever get a child with peanut allergies, then we'll all have to help protect the child by not sending peanut products, so check with the school.
I also send lunchmeat and cheese, sometimes lunchables, sometimes turkey dogs, sometimes leftover chicken and rice. I've also done boiled eggs. Last week I sent her with chicken soup, but I try not to do that too often because their teachers microwave food that needs to be hot. I haven't sent her with a thermos yet; I'm afraid to have her handle anything like that that's not cold, but maybe in a year or two. (Because we're not allowed to send reusable drink containers, I'm not sure if we can bring thermoses anyway!) I do send fruit each day.
Here's my one issue: I cannot find inexpensive regular milk to send in her lunch box. We are not allowed to send sippy cups for health reasons, so I always have to find disposable drinks. The only white milk I can find is organic, and is out of our budget when it's not on sale. I can buy little bottles of chocolate milk for half that price (two brands), but neither source sells individual-sized white milk. Why not? Isn't it cheaper to sell regular milk than to chocolate-ize it? I really don't want her to get used to chocolate milk that much. I do send little bottles of water sometimes. Juice boxes are just a treat and I put them in only occasionally.