A blanket, umbrella, first aid kit, torch, tyre inflator, de-icer, duster, scraper, map book, phone charger, emergency triangle, water (just in case the washers need topping up), small bottle of oil, crook lock, pen, tissues and some mints. Oh, and my shopping bags for the supermarket.
I keep everything tidy by putting most of the above in a zip up bag in the boot, the blanket, brolly and map book live on the back seat, the scraper, torch, duster, tissues, mints & pen in the glove compartment or side holder in the door.
I may still have a pair of wellies in the boot, too.