There are some great suggestions here. One thing I've tended to add is a breaker bar with appropriate socket for lugs, because the small wrench in the spare tire kit often doesn't get the trick done. In my wife's vehicle, I've upgraded her to a cordless impact. Let the wrench do the work.
I've replaced my jumper cables with a lithium ion jumper pack. I've used mine with other vehicles a couple times with good success. A safety advantage is for a roadway breakdown, aligning the vehicles to accommodate cables, possibly putting vehicles into the roadway or maneuvering around the shoulder, is eliminated. There are a number on the market, I have the NOCO GB40.
Basic wrenches, sockets, pliers, cutters, screwdrivers/Torx drivers. Scott brings up a good point on tire plugs and a compressor.
I'll recommend that those carrying a first aid kit be up to speed on basic first aid. The tool is only as good as the knowledge of the person using it.