Tank - don't forget the most important part of all, another person to go with. A tow strap will do you no good if you have no one to tow you out. So far I have never used a first aid kit off-road but I bring one anyway (hey you never know!). Some other things I just keep in my vehicle at all times for safety's sake:
a fire extinguisher (not that I think my exploder will catch fire, but someone elses might)
tire repair (a can of fix a flat, it'll work great if you poke a nail through it but won't help at all if you've just gashed your tire)
as many spare parts as you can bring. Usually I only have room for a few things - set of brakes, u-joints, etc.
spare fluids - brake fluid, motor oil, power steering fluid (just because my new AGR unit is leaky and until I find a replacement that's what I'm doing), maybe even some ATF (can double as power steering fluid if necessary).
Since I have a winch I always bring my winch accessory kit, which includes a snatch block, another tow strap, gloves, and that part I can't think of the name for... it also has a tow chain with it. I usually bring a 20' tow strap with me just in case though, you never know, you might have to be winched backwards.
I usually bring as many tools as possible, this means all my wrenches, sockets, plyers, hammer, etc. I also bring my battery-powered drill and drill bits (hey, you never know right)
a crow bar (will come in handy more often then one might think)
Plenty of towels/paper towels/tp
Of course I usually bring my air bed with me just because it fits in my box. How do I hold it all? Well its not quite as intricate as Rick's custom box but it seems to do the trick so far. Check it out:
and
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Matt Adams
94 Explorer 4-door nick-named "Tippy"
http://bart.is-s.com/~explorers/explorer/explorer.html