Pasta salad, which DCUM considers tacky, is the best answer. Unlimited options for ingredients, easy to package in individual portions, good cold or room temp, filling. Complete meal in a Tupperware.
I like Greek inspired pasta salad -- feta, chicken, olives, red pepper, cucumber.
You can also do rice instead of pasta. I did a really good Southwest rice salad with black beans, chicken, veggies and cumin/lime vinaigrette.