I'm surprised no one has mentioned hand tools. I keep a small tool box with the most common size wrenches and sockets, along with screwdrivers, pliars, and wirecutters. It saved my bacon a few years back. I broke the alternator belt on my Bronco II while deer hunting. Fortunately I noticed that the AC belt was the same belt, and I was able to swap it out in about an hour. Without the tools it would have been a long walk out.

I include bailing wire for impromptu exhaust repairs as well as a cheap tire plug kit from Wal-mart. Some wire connectors and electrical tape are in there too. With this stuff I am pretty much equipped to fix anything up to my level of mechanical expertise (which admittedly is not that high!)

Come to think of it, I'd better buy a spare serpentine belt for my Dodge truck today.

Regards, Vince