I actually will stop and help stranded motorists if I have time. So far most breakdowns I encounter seem to be mainly tires or fuel related(out of gas). I have added water to a dry radiator system before.
People that are involved in minor traffic accidents often have body parts that want to drag on the road and could also benefit from Duct tape, zip ties, or mechanics wire to hold those loose parts up until they get home. Another good addition is some sort of tool to cut off some dragging parts that are damaged beyond repair. Here's an example. One time a Lady had a very minor fender bender in a parking lot in her new Jaguar. She had a front anti collision sensor about 1" in diameter that broke loose and recessed into her front bumper about an inch. As a result, the car went into limp mode and wouldn't go over 15 miles an hour. If I had it with me I could have take super glue or clear packing tape and solved her problem.
For my own vehicle- I have AAA and would certainly call for a tow for anything that couldn't be fixed in an hour. On the roadside I would be willing to change a serpentine belt, jump start or change a battery, plug or change a tire, and change my CPS in my diesel, which just requires a single socket,extension, and ratchet. I would also be willing to patch a radiator hose to get me home too. I carry a spare serpentine belt, camshaft position sensor, and fuel filter for my truck
So for most people a very basic set of tools, wrenches for your battery, jumper cables, a couple of hose clamps, zip ties, duct tape, a small empty gas can, maybe an emergency hose repair kit, tire plug kit and a way to inflate and you're good to go.