If you are like most people, when you fill up the cooler, you go and buy a few bags of ice to keep things chilled, since most people do not own enough reusable ice packs to keep a large sized cooler cold for many hours. But rather than buying bags of ice, or spending weeks repeatedly filling your ice cube trays, there is a much easier way to do it.
Start saving up gallon bottles, such as from juice or milk. Wash them out very well, fill them ¾ with water, then place them in the freezer. When you are ready to go, just pop the jugs of ice into your cooler.
There is also the added bonus that you can drink the water as the ice melts, saving you from buying bottles of water on the road or drinking water from different places that taste a bit different from what you and your family are used to.