Block ice beats cubes for 5-day coolers

Block ice. Nothing else comes close for long trips.

Pre-chill your cooler the night before (stick a bag of ice in it, then dump that ice before packing). Make your own block ice in milk jugs or Tupperware—it melts slower than cubes because less surface area. Pack food in reverse order: things you need day 4-5 on the bottom, day 1-2 on top. Keep the cooler in the shade, open it as little as possible, and throw a heavy blanket over it at night.

You can stretch five days easily with block ice. Cubes will be lukewarm soup by day three.