Temperature sensor for cans and bottles
This one is a typical party problem: the fridge is full of drinks, and on the outside they are all cold. But in reality, some have only been placed in the fridge 20 minutes ago, and you don't really want to pick one of those where the drink inside is still fairly warm.
The solution: put a temperature sensor on the can/bottle with a thin wire towards the middle of the beverage. It would work like the modern batteries which have a power sensor built in. Of course the material used for this one would need to be REALLY cheap, so not sure if it's feasible.
Reward: Free beer.