Use insulated coolers for perishable items.
Before being stored in the refrigerator, cool any foods after preparation to room temperature to be served cold later.
When ready to head out for a picnic, pack ice into the bottom of the cooler and cover it with the food containers. To make ice last even longer, first place a bag of dry ice on the bottom of the cooler, cover it over with wet ice and place the food atop of it. Keep the cooler lid closed whenever not in use and store it in a shady location upon arriving at the picnicking destination. When returning home, if there is any ice left in the cooler, leftover items are considered safe to consume; if the ice is all melted, the food isn’t safe to eat. Reusable ice packs are also nice to use because they forgo the need to purchase new bags of ice each time.