Pot-in-pot invention.
Here's how it works. You take a smaller pot and put it inside a larger pot. Fill the space in between them with wet sand, and cover the top with a wet cloth. When the water evaporates, it pulls the heat out with it, making the inside cold. It's a natural, cheap, easy-to-make refrigerator.
So, instead of perishable foods rotting after only three days, they can last up to three weeks.
Brilliant ideas don't need to be difficult to execute.