Oh man ... we did this once before... And, after I got done people were askingh if it would still float...LOL
First aid kit with an airway, space blanket, bug spray, sunscreen, binoculars, matches and fire starter, flare kit, extra set of car/truck keys (and spare boat keys are always in the truck), recharge kit for my SOSPENDERS, tool kit has electrical repair stuff, connectors, etc., spare bulb and clamps for the fuel line, spare fuel filter, rain suits and a few of those one time ponchos for guests, bottled water (2), energy bars (2), extra meds for me (one of everything), three spare plugs (yup three, I'm paranoid!), flashlight, spotlight, extra AA batteries...
In the truck, for the boat, a couple of those bicycle taillights that flash to put on the trailer if my trailer lights fail me, a real hydraulic floor jack, a small jackstand, wheel chocks, road flares, a portable compressor 12v and a couple of 12v extension cords for the compressor, an orange rotating beacon and triangles, a real first aid kit, two fire extinguishers (ABC), extra oil, tow strap, tools (lots of them), 12 v work lights, a 12v splitter so work lights can be set up away from the truck, a small 110v inverter, blanket and pillow, another sorta rain suit, another swis army knife, another multi-tool, duct tape, a small pipe cutter and six 5' pieces of conduit (unbelieveably long list of uses from tent poles to splints), and the list goes on....
Most of it is the same stuff I carried in my squad car (nice big suburban) so I got used to having it and just feel naked without it all..