I carry in each vehicle:
flashlight, work gloves, Jumper cables, duct tape, electrical tape, zip ties, bailing wire, hose clamps, 12v inflater, tire patch kit. I also carry a poncho and sometimes a foam pad or blanket so I don't get dirty if I have to change a tire.
In my wife's vehicle I have a multi-tool and some basic screwdrivers
In the truck and the vehicle I drive I have a basic tool kit with a breaker bar, the double ended lug nut sockets, wrenches, sockets, pliers, screwdrivers, multi-meter, and a slew of other things.
yeah I know I won't be doing major mechanical on the side of the road. These kits have been used more at family and friends houses to help them with stuff.
Plan for things you're likely to encounter: jump-offs, changing tiers, patching nail or screw holes, and maybe patching a radiator hose. I would consider changing a broken belt, a thermostat if it's on a truck of something with easy access, or doing some other emergency repair to get the vehicle drivable until I can get it back to my shop.