Water sounds like a good suggestion, maybe not this time of year. Not sure where the OP is going, but survival sounds good. Same vein of thought, I try to keep one or two moving blankets in my car (cheapo HF ones). Toss on the ground in case I need to do work, or toss over the backseat if I have something greasy to move—although I bet in a pinch they might work as a blanket too.
I’m kind of a fan of the cheapo tool sets that come in a blow molded case. Commonly used sizes. Maybe buy a better ratchet, usually they come with junk ones (but wild guess here, you’re not the type, you’d buy a better set of tools in the first place). Just good enough for basic roadside repairs, anything that can’t be done with those tools is a job probably best not done on the side of a road. I mean, until you start packing a welder, a couple of batteries for your electric impact wrench, and a 6 foot breaker bar… at some point, it becomes too much (unless if you‘re one of those hardcore off road guys, but then you’re never out in the woods alone, right?).
Maybe get one of the better crimp tools and some butt splices. Ring terminals. Vice grips of course. Various kinds of tape (electrical and duct), spare wire, zip ties. Spare fluids. Tire inflator?
To be honest, I have to wonder, if something goes, you have to get to a place to buy the replacement part. Buy the tools then, maybe? How many places are going to have a car parts store yet not a Home Depot / Lowes / HF / whatever.