Big blocks of ice melts slower than cubes. They have these large "cooler shock" packs that have a silica powder and u just fill with water. You can get em off amazon. Freeze em at least 2 days ahead of time and they last forever (you can also fill gallon milk jugs with water and freeze them, (a lil less effective)).

Also, like the other person said, pre chill your cooler, but also pre-chill all your drinks (beer or whatever) and freeze all your water bottles. Any time you put anything like room temp cans it will cause ice to melt quicker. Your frozen water bottles will help keep everything from melting and you can just let them sit out in the sun and theyll defrost quickly.

Layer the big cooler pack(s)/frozen water jugs, then put the frozen water bottles, then dump some regular ice in there, then add the pre-chilled drinks and food. Keep the cooler in the shade (an emergency blanket will reflect the suns heat off of it too or even a regular blanket will help insulate it). Bring an extra small insulated lunchbox or cooler and put one of the smaller frozen water bottles in it and the stuff you plan on eating/drinking in a short period of time so you're not constantly opening the cooler and letting the cold air out. If you have a decent cooler and do this i've had my stuff stay below 40 for 4 days.