Ditto on the fire extinguisher. If you have a turbo explode or drive over spilled gasoline or propane, you may need a fire extinguisher to kill the engine. I live close to Eugene and there are still lots of unburned VW Bugs and unburned Harleys to put out.
For those that only plan on carrying a credit card and cell phone... you may find yourself out of range and with no cash. Many times you may have a good Samaritan with the knowledge to fix your problem, but no tools or parts. When I stop to help someone and they have made no plans to help themselves, no tools, no spares, poorly maintained vehicle, I may not put out a full effort.
Key Items from my list:
'Truck Air' 12v air compressor ($35),
4 way lug wrench (mostly for trailers and others),
Glass Cleaner (works for hand cleaner, parts cleaner)
Fuses (My 01' has about 12 kinds)
Hi Temp RTV sealer,
Wire crimpers, crimp connectors and good 3M #33 electical tape,
Usual tools, oil, filters, belt, wire, jumpers, tire plugs (lots).
For off hyway, I just plan on camping:
Flashlight, 5 gal water, sleeping bag, shovel, wagon jack, wire, 2x12's, propane bbq, food, tow chain, snow/mud chains, u joint, extra spare (used it), CB, 12 gage pump with 00 Buck and BBs.
If you plan for a problem, you won't have a problem. Pessimists are never disappointed, they are pleased to be right and happy to be wrong.