You can buy the "Cold and Hot" packs at Wal-mart or Canadian Tire. Boiled the water, turn off the heat, put the "Cold and Hot" packs in this boiled water for 8-10 minutes, then take it out, dry the water up, put with the lunch container. This way, when it is the lunch time, the lunch is still warm. If you want the lunch even warmer, just put one more "Cold and Hot" pack.
Another tip: buy double deck lunch bag, that way you can separate the cold snacks from the hot lunch.