Last nights leftovers
that's my lunch a few times a week. i especially make sure there is an extra portion when cooking dinner. i also take salads, soups, sandwiches. i sometimes have crackers/rice cake with some sort of dip like tuna, egg salad, chumus, chatzilim, cottage cheese, etc...
i also always have nuts and candy in my desk JIC i decide i'm not in the mood for my lunch or forget to bring.
worst case, i get something from a local restaurant. i usually ask coworkers if they want anything so i can meet the minimum for delivery.