The Maglite isn't a bad idea - mine is clipped in on the D pillar at the rear of the vehicle. A small fire extinguisher is between the driver's seat and the doorsill, and a larger one is on the other D pillar.
Why is the Mag in the back? Because there's always a SureFire G2 on my belt (I'll be converting that to LED as well one of these days - just haven't done it yet. The original bulb is still going fine.) Check the batteries in your Mag about every quarter, as conventional alkalines do drain.
Check your local Army/Navy store - I've found a batch of 3-gallon rugged plastic water jugs, and one or both is filled with "purified" water (the stuff from the machine.) It keeps a bit longer, and you can use it in you or in your truck.
A folding shovel will probably see a bit more use than a straight shovel - not all of your digging jobs will be that large. Carry both.
An ax is nearly always useful - either to cut wood, or as a hammer.
While you're at the Army/Navy store, get a few bottles of "lifeboat matches" - they're essentially "strike anywhere" matches dipped in paraffin wax. Lighters can run out of fluid - those matches will keep for just about ever. Massage some petroleum jelly into some cotton balls and put them in a baggie or a waterproof bottle, as they make great kindling and firestarters. Pill bottles are great for keeping these things in.
A good fixed-blade knife, to keep under the back seat. I like the M7 bayonet or the Mk3 Mod0 diver's knife - both of which should be available for a nominal price at the Army/Navy store.
A box of spare fuses and lamp bulbs. You should pad the bulbs, but the ATO/ATC plastic blade fuses are difficult to break without some serious effort.
A rope - ten metres or more. Amazing how that can be useful sometimes...
A blanket - in case you spend the night in your truck.
I also keep an old T-10 parachute canopy in the truck - tent, truck cover, sleeping bag, or any of a number of other things. It's usually a bit spendy, but can be well worth it.
Quite a few other things can be useful, but are also specific to various environs (you don't need the plastic sheet and cup to make a solar still in the forest - water is easier to find there.)
Oh - and see if you can't find an old Boy Scout Handbook in your travels. Bloody useful book...