Ok I don't think I'm as organised as most people but I do fill the car with stuff
Boot - Sun tent(it folds flat), buggy, 2 folding chairs, over the shoulder baby holder/walking thing, 2 headrests (so that the kids seats fit in), first aid kit, couple of bottles of water (for car) rain cover for buggy.
Rest of Car - Wet wipes, tissues, umbrellas, toys, books, travel booster seat, potty, rugs, haynes manual, tapes, odd receipts, elastic bands (from collecting the post and not taking the elastic bands in) book i'm reading, more bottles of water.
I normally manage to take out 1 bag of rubbish each time I clear it out and then a couple of bags of stuff that is no longer needed in it.
I think mine is like a huge handbag and I could never pack light