In the under-seat storage in the rear, I have a pretty complete mechanics tool set, including wrenches and sockets, various sizes of screw and nut drivers, three kinds of tape (duct, electrical, and metalized), pliers, electrical components (packard connectors, etc), and assorted spares (lamps, belts, etc). I also have a small handful of nuts/bolts/screws. A couple of hose clamps round out the kit. This is all stuffed into a waterproof roll bag with desiccant, since it gets wet back there. The first aid kit and fire extinguisher that go with the truck are also installed--though I admit to beefing up the first aid kit.
Also on the truck I have a set of pioneer tools, towing straps and shackles, chain, a jack, lug nut wrench, and so forth in case I get stuck or a flat.
On my list are a roboimpact and 24v air compressor. But that's for later.