In addition to the typical spare parts and fluids, I carry a better jack. found in most 90s Dodge trucks and zjs and xjs is a nice under the axle jack with a crank handle. Better than trying to completely unload the suspension to change a flat. Also have a soft side tool bag with the essentials. A basic inexpensive sae/metric socket set and boxed/open combos, screwdriver with changeable bits, multimeter, extra wire in a few gauges, strip/crimp tool, tape, fuses, and the big one.
A headlight. Nothing like being able to see what you're working on while using both hands to do the job.
Of course, you'll be stranded at the most inconvenient time. 0-dark-thirty-in the rain-middle of nowhere-etc. Thought about a makeshift creeper (think cardboard or tarp) to keep me clean enough not to ruin the interior once I get back in the Jeep. But the headlight has been a hands free lifesaver. many times.
After that, a cell phone.