The spare key to your car. I keep mine on me in my trouser pocket in case I lose my keys while I am out. Silly because I keep my main keys ... in my trouser pocket. Unless I lose my trousers I should be okay.
I keep spare one in handbag hubby keeps main one in his pocket. I dont drive, so mine hardley ever comes out of the bag. But as he is my carer we arnt very often seperated so if he did lose his we would be ok.