Extra ice for the extended stay.
Enough food for the time out and some extra rations that don't require refrigeration, for emergency use. Pre-prepared sandwiches, cold pizza, fried chicken and things that can go bad should be used up first.
Extra water. The amount needed per person varies, but in warm weather, a gallon per person, per day is a good rough guideline. You'll need more if temperatures are high, and this does not take into account washing, only drinking water.
Extra bait, should it be needed.
Extra fuel or full reserve tanks just in case. Figure out your fuel requirements for the trip planned. Use the standard guideline of allocating a third of your fuel for going, a third of it for the return trip and a third as reserve.
File a float plan with family or friends, and let someone know when to launch a search if you haven't checked in.
Ensure that engines are in top working order.
Ensure that all electronics are in good working order.
Pack foul-weather gear and a set of dry clothes — these can make or break a trip.
Take extra illumination and spare batteries.
Bring a satellite phone.
Keep a spare handheld GPS in case of a major malfunction.
Pack charts in case of an electronics failure.
Maintain a well-stocked first-aid kit.
Bring any required medications (blood pressure meds, insulin, etc.) for those who may need them.