I agree with everyone who's mentioned leftovers. I also:
~ stock up on canned soup w/ the pull-tab on the lid when it's REALLY on sale...DH likes these because he can be filled up on one can (the little single-serving or hand-held cans are way more expensive than the others). Just remember to take a bowl or large mug and spoon with you!
~ Buy ramen noodles--you don't have to use ALL the sodium-laden flavor packs...Just a little goes a long way. You could also add a bit of peanut butter to the noodles for a Thai-style meal.
~ Make a list of 10-15 main dishes like PBJ, Turkey Sandwich, Ramen noodles, soup, chili, etc.; plus a list of 10-15 side items like apple slices, carrot sticks, cheese sticks/slices, etc.; plus a list of 10-15 treats like leftover Halloween candy, pudding cup (that you make at home), applesauce, etc. Then make sure you have the ingredients on hand for 80% of each list and you'll have a ton of options to pick from for lunch!