hit the halloween treat aisles NOW. get the healthier treats and use those for lunches. so far i've gotten tiny bags of mini oreos as well as tiny bags of welches fruit snacks (in the past i've found graham bears and cheese gold fish). the per unit price is great and the kids love the mini versions of their favorite snacks.
as far as cold stuff goes-pb sandwiches (or goobers as ds insists on), meatloaf sandwiches (or the alternate 'meatball sandwich' which just entails me spooning some jarred spagetti sauce on the meatloaf for the sandwich

), bologna, lunch meats...string cheese sticks, sandwich wraps (if they like cream cheese use it in place of mayo).
snackie stuff-craisins, raisins (and ants on a log), trail mix, banana chips, dried mango, quacker mini rice cakes (dd likes the carmel flavored), chips, crackers, 'veggie sticks' (not the real kind-they're like those old shoe string potato snacks but made out of veggies and come in a variety of colors-even my non veggie eater ds enjoys these).
i agree with the thermos idea-dd lives for leftovers so her lunch is largly dependant on what's in the fridge from the nite before. ds on the otherhand pretty much rotates the same 3 items for lunch day in and out-goober sandwich, top ramen (they have access to a microwave) or baked potatos (i prebake then split it and put the butter and cheese on it-he nukes it in the container i send it in and sprinkles it with a little container of bacos.