Good question. I carry a 12v compressor which just fits in the tool tray, a small rectangle of wood about 3/4inch thick to spread the load of the jack on soft ground, and a couple of relays. I also carry a standard tool roll with screwdrivers, spanners, pliers etc, and a few cable ties & spare fuses & bulbs. I also keep a sheet of paper in the glovebox as a reminder of what all the fuses are, and a small torch. To be honest, apart from a flat tyre, there's not much scope for a DIY fix on the roadside.
