Here's DD's lunch setup. I posted about this back in June when she first started preschool and I started packing a cold lunch.
Although my photo shows sandwices, it turns out DD doesn't like pb&j. Here's what I usually pack in a lunch:
ham cubes or ham slices cut into shapes like hearts, starts, flowers
spaghetti, marinara sauce, ground beef or gound turkey
plain pasta with parmesan cheese
carrot sticks (If I have a really big diameter carrot, I use a flower veggie slicer)
cheese sticks or cheese slices cut into hearts and star shapes
I do rice balls with chicken, pork, or ham cubes all the time. However, it's not warm. DD's school does not heat up food. DD still loves her cold rice balls and I tend to send very simple meat dishes that don't have a lot of sauce or butter, which wouldn't taste good cold. Sometimes, if I feel artistic, I decorate her rice balls with little funny faces made of of seaweed. If I send a rice dish that isn't in a ball, it's fried rice. Cold plain rice is just not yummy.
I send try to keep dry foods like crackers or yogurt chips in a separate container from the bento, because humudity from other foods often makes them soggy.