This is a nifty little trick I picked up somewhere, not sure where. Instead of buying those squooshy freezer gel pack thingies for keeping school lunches cold, take a snack sized ziploc, fill it with dish soap (like Dawn or Palmolive) and place in freezer. Then add to your kid's lunch bag. It will keep things cold until lunch. Have the kid return it and reuse indefinitely.
As an alternative, you can also freeze your kid's juice box or yogurt cup; as it thaws it will keep the other food cold as well.