In my car boot I have a small holdall with: a first-adi kit, an old army blanket (for warmth), a sheet (for picnics), a handful of nappies, a couple of muslins, a handful of small toys and boardbooks, jumpleads and sunscreens (for windows), some carrierbags.
When pg and as long as breastfeedign afterwards I would keep a large bottle of mineral water in the boot, and a small bottle with sports cap in the front of the car. I never drank from the large bottle, so that the water would remain fresh, and the children learnt quite early to drink from the sports cap.
In the front of the car (under seats or in glovebox or door-pockets) I keep a fire extinguisher, torch, notepad, pens, some more tiny toys, indigestion tablets, a little 'dustbin', chammy, ice-scraper, box of tissues.
All these things have been used at one point or another.
Things like change of clothing, cup and bib, etc I kept in the changing bag - after all, until a certain age you don't step outside your front door without it!