Old Car Antenna Aerial: Available in most scrap yards or along the side of the road for even less, the old (without wrapping of additional wire) stainless steel car aerial can be cut and bent into a reasonable spring clip or retention pin in an emergency to replace whatever just flew a few feet from the rig and into the grass to disappear forever!
Paracord, 550 Line, Small Stuff: Whatever you call it, the day you leave home without it is the day that you need it. Best stored in it's own small bag! Learn how to properly seize the ends while you're at it!
Grill, Charcoal and Chimney Starter: For S'mores, steaks and general cooking on the day that BOTH the LP tanks are empty because you forgot to remember to fill the empty one up.
Hydraulic Bottle Jack: For assisting with changing tires, leveling and stabilizing after you clip off one of the mounted stabilizers on that rock next to the road on the way to the primitive site and for assisting the other person who now needs to borrow a jack.
Hose clamps, various sizes: Sewage hoses aren't the only things that break!
Two Part Epoxy: While I prefer the ribbon type, the mixable can type works and stores well too. For those days that you find that there's a leak that can't be fixed with the hose clamps.