I keep finding pairs of knickers. Not sure where they come from, but they're great for cleaning the PCM screen
But seriously:
A Roll of soft kitchen towel and a spray bottle of AutoGlym Fast Glass, kept behind the seat, for on-the-spot bird poop removal.
A compact map of the UK that fits nicely in the pocket behind the seat. Had to go buy a small map though!
Handbook, pen and paper in the glove compartment. Should have a disposable flash camera in there as well in case of accident, but a lot of people have cameras on their phones these days.
Mobile phone charger and electronic tyre pressure guage in the cubby behind the handbrake.
I keep my CDs in the driver side pocket. I only keep CD copies in the car, and put them in ring-binder cd sleeves. You can make a little flip-book of them with some treasury tags or ring-binder rings. Any of the soft CD wallets are just too big I think, so I had to make my own.
I also keep a small packet of wet-wipes, for wiping hands post sun-screen application.
And obligatory packet of mints, and some pro-plus. I think I've got a bottle of water behind the seat at the moment too.
Oh, and I've lined the rear boot with a rug, to keep most of the dirt off. I often go to the DIY store or garden centre. It bemuses staff when you ask them to put a couple of bags of compost in the back.