i work in sales and find a two prong approach works best.
the first prong is always having shelf stable snacks in my bag or already in the car. they make pearsauce, applesauce, fruits in their own juices or gels, fruit snacks and leather, dehydrated fruits and/or veggies, granola bars or bags, nuts, individual nut butter packets, individual hummus packets, cheese/nut butter and cracker packs, tuna salad and cracker kits, and shelf stable cheeses to name what i can think of offhand. check the lunchbox and snack aisles at the grocery store to see what you have access to. yes, you have to read labels. but i have three or four of these things in my car or bag at all times and a decent supply in my cabinet as well.
the second prong is packing just the right amount for that day. the idea is to not have to bring home perishable stuff that i didn't eat, but to not have to rely entirely on my nonperishables [though it's great to be able to do that if i leave my packed lunch in the fridge that day]. this is where you bring yogurt and berries [and possibly add some granola from your stash]. i will say i love my thermos funtainer. it hold ten ounces and keeps stuff hot or cold for five hours. on days i know i'm going to be long and that i'll need to eat late, i'll portion in my yogurt, then add frozen berries.it's thawed enough to eat five or six hours later, but still insanely cold. it's where you bring soups and stews, sandwiches, leftovers or cheese and crackers. you're only limited by the container you bring your food in. so plan to eat the most perishable thing as a morning snack or early lunch, and then the hardier things later in the day. so a good thermos, a good insulated lunch bag [target sells a decent one for under $5] and a few ice packs and there are few foods you can't bring [crepes and fussy souffles being the only things that come to mind offhand].
-google first. ask questions later.