This, from one of my best and oldest friends, Judie (always a thrill when I discover a friend reads Parent Hacks):
I usually use Blue Ice, which is an ice substitute that can be frozen over and over again and is packaged to not leak, in my insulated picnic cooler to keep perishables fresh. I also freeze water bottles filled with water when our family is going on a road trip so as the ice melts we’ll have cold water to drink. I discovered that if I freeze my kids’ water bottles overnight I can use them in place of Blue Ice in their insulated lunch bags. By lunchtime at school they will have cold water to drink and fresh non-spoiled food to eat.
Also, two-liter soda bottles are also a great substitute for Blue Ice and ice bags for your big chest coolers. Just freeze the water in the soda bottle and you have generic Blue Ice. The ice melts inside the bottle and doesn’t get the food items wet. It also stays frozen longer than ice cubes.