I don't holiday any more, but used to take so many 'just in case' items that there was little room for clothes. I took most of the things mentioned above, (definitely loo roll) plus a variety of glues and sticky tapes, veg knife and a small non-stick pan with spatula if self-catering, a blow-up cushion for airport delays, tools, universal bath/basin plug, portable bidet, washing line and much more.
For every-day car trips I have a permanently half-full boot, containing snow shovel, folding sack truck, tool kit, wind-up lantern, shopping bags, picnic blanket, wellies, foot pump, bottle of water, loo roll, old fleece jacket and lots more, all covered over with an animal print throw. Oh, and a Shewee with gel bag in the glove box!
I am definitely an 'everything but the kitchen sink' person!