Teri's good-thinking grocery hack:
When I go grocery shopping with my toddler, I keep the cold or frozen items together in the cart and at checkout. That way, I end up with nicely sorted "cold" bags. (The items keep each other cold as a bonus.) If my little guy needs some attention when I get home, I put away the one or two cold bags and leave the rest. Boxes and cans can wait. This lets me take care my little guy without stressing out about what's melting in the car. It was also helpful when I was pregnant; most of the groceries could stay in the car until my husband got home. In the winter (living in Canada), switch the hack to separate items that can't handle being frozen (e.g. lettuce).
Trader Joe's sells a fantastic quality insulated cold bag for $5 — heavy-duty red and black canvas with a thick vinyl interior. I keep mine in the car for groceries and cold snacks and drinks. When I'm really organized I remember to throw in an icepack before I leave the house.
Related: Smarter grocery shopping