Right now in my trunk I've got:
A radiator
A glow plug relay
A rear view mirror
An aux fan
A shut off actuator
A VCV
Two turn signal housings
Plus a full socket set, a tool box and another tool roll full of tools
BUT that is because I was just pulling stuff at the junkyard and haven't unloaded it yet
Normally I don't carry much.
Fuel filters. Voltage regulator. Some fuel return line. Basically just a few things to fix stuff that can suddenly and without warning strand you somewhere but that are so easy to fix you'd be irritated to have to tow the car as a result. And of course the tools needed to do the above, and change a tire.
Otherwise, for anything more, I just have a tow service through my car insurance.
Then again I do lots of preventative maintenance so the plan is to not break down!