One of our family's favorite jokes centers around the Thermos. It keeps the hot things hot and the cold things cold, but how do it know?
Ok, so maybe you don't find that as funny as we do.
When it comes to picking up the best grocery deals an ice chest in your trunk is a great way to keep your perishables cold all day. You can slip into a nearby grocery store in the morning, pick up all the great deals before the store gets really crowded, and then keep them cold all day while you are out running errands or working.
Before you think that you'd also need to buy some ice to keep things cold, rethink that idea into a free option. I keep a a one gallon jug full of water frozen in my freezer (and an old 2qt apple juice bottle too in case I want something smaller) and when I know I am going to pick up some deals that morning, I move the jug into the ice chest.
The night before you plan a shopping trip put the ice chest in your trunk. If the temps are cool enough at night where you live you can put the frozen jug in the ice chest at night too so you won't forget it in the morning.
Just like when you are camping, ok maybe you don't camp, but other people do and they use the ice jug trick to keep their cold stuff cold without it all getting soaking wet and swimming in water after 24 hours.
Or you can think really old school and think of an ice box. Ice in an insulated container. Yep, keeps the cold stuff cold. It 'knew', because it was properly named.