Now that I have worked out some of the glitches in my 240, I carry the following:
a spare fuel pump relay
a spare distributor cap
a spare distributor rotor.(this fried once and left me stranded).only use Bosch
a small spool of bailing wire...fixed a flopping radiator shroud and a broken muffler mount
a package of zip ties...the type with the metal zipper are the best
a roll of duct tape
a tube of quick setting binary epoxy (i like the type that comes in a tandum syring with a mixing tip.
I believe that with some thought...the preceeding 3 items can fix 99% of the worlds problems
a can of spray lubricant like pb blaster
at least a quart of oil
a quart of ATF - (i have a persistant power steering leak)
a pack of road flares
a selection of worm style hose clamps
a pair of leather gloves
a jack
spare tire
a 12 volt air compressor
a tire guage
a tube of RTV silicone
a length of strong rope or in my case a piece of 2 inch wide nylon webbing
a leatherman style multi-tool
ciagarette lighter.
an 18 inch pry bar (don't ask)
Since I live in Pittsburgh, aka the road salt capital of the US, in the winter time i carry an extra gallon of windsheild washer fluid....once on a trip from pittsburgh to dc i used nearly 2 full tanks of the stuff...and it was the good stuff! (oh yeah and at 0 degrees F..the cheap blue stuff freezes solid here)
I also keep a full 200 piece craftsman socket/combination wrench set in the trunk. yeah it is overkill, but i like the idea of it....plus the old brick has used it a time or two in rescueing some of my associate's much fresher cars.
Almost all of this, with the obvious exceptions, can fit into a box about the size of a milk crate if you pack well. good luck.