This is a super easy and inexpensive way to keep your food cool while out on your summer adventures. All you need is gallon sized freezer bags. Fill the bags about 3/4 full so that when they lay flat they are about 2 inches thick. Place flat on a flat surface in the freezer. You can stack them two or three deep. More than that and they will fall over and freeze in a blob instead of nice flat sheet.
Step 1: Freeze Flat
This is a very cost efficient way to keep your cooler cool. Just place filled bags flat in freezer the day before and you will be ready to go. The flat solid block stays cool longer than ice cubes and fits nicely in between the items in your cooler.