Use an insulated lunch bag. The ones I use have a removable inner bucket and I put the food inside it and then place in the fridge. My kids may have different things in their lunches so I have put their names on the outside of each bucket. You may forget what you had put in each bucket. Use small freezer packs that can be refrozen every night.
Nothing is worse than a cold lunch pretending to be a warm one.
If you don't have freezer packs, freeze a juice box overnight and then put in the lunch bag. It will keep the contents cool and will be nicely thawed by lunch time. Wrap lettuce separately in a baggie with a sprinkle of water. It keeps it nice and crisp. If you run out of lettuce for sandwiches, use sliced pickles.