In the cabin - nothing. No junk allowed. No food for the kids. iPod in the glovebox wired in permanently. Yoyo in the door pocket.
In the boot - I believe there is a spare tyre (there was when I got the car), there are three laptop cases (all for different uses - traveling etc.) and heaps of paperwork that should be somewhere else.
I once carried most of the parts required to rebuild the car if everything under the bonnet was vaporized by aliens, but I now know there are tow trucks and few times that I can't afford to be late.
If your car is that mechanically challenged that you fear you need a spare engine, get a VW Beetle - there's a spare in the boot.
There is a difference between daily suburban travel and remote travel - if I know that I'll be miles from nowhere (logically that puts me somewhere) I will carry a small quantity of water, oil, basic tool kit etc. Round town there are enough people who can be contacted to provide "assistance".