Id ex the fire extinguisher being inside the tool box, by the time you find the thing, get the key, lean over, move stuff, free it from who knows what .. you wasted some serious time.
I have a tool box with 5-10 of them in it at anytime and I could honestly say it would be a hassle to get one out considering the fact that I have to lean over and in, open the lid.. then any of the other factors above. Get a couple mounts and put them near your seat frames.
OK.. now onto the actual question
-couple gallon jugs of water "if I break down in az ..its needed"
-gallon of ATF..line breaks, hole knocked in pan..all happened before
-gallon jug of oil.. swamped the motor in water..
-bag with spare cap/rotor and ign module couple spark plug wires
-few feet of 5/8 hose, 3/8 hose and spare radiator hoses with some clamps,
-spare steering rag joint and centering pins
-Tow strap, d-rings and tree saver
-come-a-long..pull a axle back into place after failed center pin
-bag with spare leaf spring bolts, misc bolts/spare steering arm studs
-bag w tire plug kits, valve core tool and spare valve stems, pres gauge
-bag w spare 1350, 1310 u-joints , straps and bolts
-bag w spare fuses, spare battery cable, and 10 and 12 ga wire/terminals, test light
-spare rear drive line bolted down with exhaust clamps
-spare front shafts with joints in a pvc tube, strapped down
-Right stuff rtv, bar soap, rope, thread/needle
-pry bar.. reseat tires, move crap, hit people, tighten a belt
-ratchet straps..hold things down, unruly passenger, reseat a bead
-1/2 and 3/8 sockets and adaptors
-wrenches, screwdrivers,pics
-hand wipes, bar soap to plug holes or clean hands, paper towels, sharpie, compass, 4-5 top roman and couple caned soups, 1 good flash light, 2-4 of the cheap 2 pack led's from harbor fright, couple cheap tarps or a blanket left in their package, small 12v compressor and a decent first aid kit.
-spare set of clothes/shoes and a towel
ALL of these things I have needed or come across someone who needed it more than once.. and it was not always hard wheeling that made them needed..
1 time while out just a few miles from town exploring the back side of a mountain I got hung up on some rocks.
By the time I gave up I had deflated my tires too much, broke a front axle shaft, broke the rag joint, dented the rear shaft, boiled over my motor and popped a lower hose..
I then had to give up and walk back. What a damned mess.. I have learned a little something every time myself or another gets screwed while wheeling or exploring.
1 thing I have not needed but do carry is a snake bite kit, I have a couple of them from wally world...and get a spare !