I'm a bit of a boy scout and usually bring the kitchen sink...
Milk crate that's always with me even when daily driving:
Engine oil, 1g
Coolant, 1g (usually just distilled water in a coolant jug, or some mix)
80w90 oil, 1qt
ATF, 1qt
Starter fluid
Wheel chock
Wood blocks (for jack)
Rags
Tools always in the truck:
Socket set, 3/8", metric
Factory bottle jack (with wood blocks to increase height or add stability)
(the following in the original Toyota bag under the seat)
10/12 Combo wrench, Toyota
14/17 Combo wrench, Toyota
10, 12, 13, 14 ratcheting/open box wrenches (13 is for a special purpose)
17 open wrench (need two 17s for the carb fuel line)
12mm open wrench, cut in half (for getting to the engine-side carb bolts or in other tight spaces)
Screwdriver, flat & JIS cross point combo, Toyota
Screwdriver, long skinny flathead (adjust idle mixture with air cleaner in place)
2x vice grips, small (sealing off fuel or brake hoses)
Hub cap remover (plastic tool that works for 40 wheels)
Pliers, Toyota
21mm socket with extension (for lug nut & spark plugs)
Paracord (for tying up the sway bar when I remove the end links for better flex)
Big bag of spares, carried on longer trips or on the trail:
Coil & igniter
2x spark plugs, used but good
2x spark plug wire
Cap & rotor, used but good
Starter, rebuilt
Voltage regulator, new OEM (sadly NLA, aftermarket ones suck)
Electrical supplies, 1g ziplock: wire, splices, shrink wrap, crimper
Brake supplies, 1g ziplock: front brake pins, 2x used brake hose (long & short), brake hose clips
Front axle c-clip
Rear diff c-clip
Pinion nuts (t-case, diff)
Regular belts, used but good
Emergency belt, non-standard size
(if smog pump seizes, remove both belts and put this around the crank pulley, water pump, and alternator - alt provides tension and you keep the cooling system going; you lose power steering but you get home)
Nuts & bolts, common sizes from large to small, 1g ziplock
Fan clutch, used but good
Radiator cap, new OEM
Engine mount, left side
Hose, various sizes for fuel, vacuum, coolant, etc
Hose clamps, various sizes
Upper & lower rad hose, used but good, with clamps
FIPG, various for oil, coolant, etc
Gasket material, small sheet
Gloves, two pair - heavy and light
Big channel locks
1/2" ratchet
Adjustable crescent wrench (doubles as hammer)
Socket kit (with larger or more rarely used sockets such as deep ones)
Ratchet straps (for when your engine mount or leaf springs brake; have used them twice to hold my engine to the frame and get off a trail or to camp)
...there's a few other bits and bobs in there, but that's what I remember from memory
EDIT to add:
Fuel pump
Drain/fill plug gaskets
Vacuum stuff: tees, connectors, caps
Fuel filter
Recovery bag, trail days or snowstorms only:
Dynamic strap
Static strap
ARB snatch block
2x ARB screw pin bow shackles
2x off brand screw pin bow shackles
Tree strap
Foldable traction aids
(the following don't fit in the bag)
Hi lift jack
Shovel, a small cheap one from the army surplus store (dug myself out of a mud pit a few weeks ago; it's also dug many cat holes!)
Portable air compressor
Jump pack (though I have dual batteries now, maybe this stays at home)
Jumper cables (still useful with dual batteries)
I really need to start packing bailing wire! I've seen that get folks off the trail before.