There's a lot of junk in my trunk.
In seriousness:
- The following never leaves the Jeep:
Compact Harbor Freight ~50 piece socket set, extra fleece blanket (just in case).
Tool bag with a bunch of miscellaneous tools for my Jeep: mechanic's gloves, tire gauge, good ratchet straps x2, paracord, extra work gloves, poncho, glow sticks, baling wire, electrical tape, multi-meter. Assorted pieces (extra D-clips for sway bar, extra hitch pin clip, adjustable hose clamps, screws & bolts). Wire cutters, channel lock, linesman, needle-nose and regular pliers. Box cutter, screwdrivers, multi-purpose screwdriver with Torx bits and extensions, adjustable crescent wrenches x3, SAE wrenches, SAE & metric dog bone wrenches, 3/8" drive T-handle, extra 3/8" drive ratchet, universal socket, 3/8" drive short extension w/universal impact adapters.
Backpack with recovery equipment in it: recovery strap, rubber mallet, tree strap, clevis x2, axle strap, rear recovery point with extra hitch pin, tire repair/sealant, tarp, sheet, safety glasses & dust masks, TP, bungee cords x4, tie down straps x2, folding shovel. Tire deflaters.
- Non-year-round items include: ice scraper, tire chains and chain tensioners.
- Basic toolset, IMO, has been the most helpful. Basic tools and some know-how will allow you to perform a ton of maintenance/repairs without a full garage. Sometimes, when starting a project on my Jeep, I'll use the tools that are already in the vehicle and see if I can perform the entire task without my larger, master toolkit (e.g. I just did then when replacing my transmission mount last month). You'd be surprised, a few screwdrivers, pliers, basic sockets, etc. will take you a long way.
I will admit, half of what I keep in my Jeep I've never used and could probably do away with (e.g. never used the paracord, poncho, glow sticks, hose clamps, dog bone wrenches, T-handle, adjustable crescent wrenches, etc. in all of my adventures). That being said, it's not like the above takes up that much space - I can fit it all in the tailgate so I don't even notice it.