Block ice is the answer.

Yes, if you can’t refill ice, block ice lasts way longer than cubes. A few big blocks are worth the effort.

Cube ice melts fast because of surface area. Block ice is dense and slow—think of it as a cold battery that also keeps things wet as it melts. Freeze water in milk jugs or Tupperware, then put them in the cooler before you load anything.

Start with a pre-chilled cooler (store it in a cool garage overnight, or toss some bagged ice in there for an hour, then dump it before loading). Also pre-chill whatever you’re packing: warm drinks are the enemy. Fill gaps with towels or newspaper to reduce air movement. Open the cooler as little as humanly possible.

If you need more than two full days, dry ice is the next step—but handle with care (ventilation, don’t seal airtight, wear gloves). For a standard weekend trip, block ice plus good prep will hold fine.

You don’t need fancy coolers to make this work. Just a little planning.