When you're filling up your esky, one necessary ingredient to keep the whole thing cold is a bag of ice. Instead of slogging your way to the nearest servo, The Kitchn recommends you simply fill up some old water bottles and toss them in the freezer.
The benefit here is two-fold. First off, you don't have to go to the store to buy a bag of ice. Second, you get to drink the water when the ice melts. Just fill up a few old water bottles with water, toss them in the freezer the night before, and you're good to go.