A compass, or a GPS (along with a compass) are a couple things one should never venture into the wilderness without. This is basic, and as far as I'm concerned doesn't even deserved to be titled "Outdoors 101" as it's rather remedial in nature. The question is; what do you consider "wilderness"? I routinely venture into the wood lot where I do my deer hunting without my GPS, or even looking at my compass for a heading. The lot is small, and I've learned that a straight line in any direction will bring you to civilization in well short of an hour. But that's my comfort zone, and each outdoors man needs to find their own. There are other places I would never go without a means of keeping track of where I am, and how to get out.
A knife and a flashlight are two items that one should never be without. A day trip can present opportunities that stretch your outing to last light or beyond, and the need for a knife presents itself quite easily. I never wander the woods without either. A couple of other items I like to carry are dog oriented. A small pair of forceps are useful after the dog encounters a porcupine, but equally handy should you suffer a misstep in a thorn apple grove. I also carry EMT gel in case a dog receives a nasty cut. I've had dogs get cut up before, and though I doubt their life was in danger, I felt better once the bleeding was stopped. A 10' length of parachute cord can double as a slip lead if needed, or used to immobilize an injured dog.
There are a number of "worst case scenario" items I carry in the outdoors, too. These are the items that, should I be stuck, unexpectedly, in the woods over night, will hopefully make the experience a bit more comfortable. As I've come to believe a night in the woods is a matter of when, not if, I view these items as essential. Though I haven't assigned them an order of importance, the moral boost, heat, and security a fire provides can't be overstated. To assure I'll have fire, I carry two means of starting one; waterproof matches, and a zinc/ flint starter. There are others out there, and some I'm keen to get my hands on, but for now those are it.
For additional warmth, and a bit of shelter, I carry two emergency blankets. The way I see it, one wrapped around me, and the other suspended overhead should keep me warmer, and dry if the weather isn't cooperating. As I sometimes hunt with friends, the extra blanket can provide them some shelter too, should I have company in my misfortune. However, they won't find themselves wrapped up in it, but sharing my space beneath it, as they should have had the foresight to pack their own blanket. Hell, I've been advising them of the need to assemble a kit for years.
A couple of other tid-bits I carry are a roll of heavy weight twine, an extra knife, a couple light sticks, and a snack of some kind. The twine can be used for just about anything, but suspending a blanket for shelter readily comes to mind. The light sticks, snack, and knife don't really need an explanation. This all goes into a big zip lock, which can be useful in a pinch, too, and then into a small stuff bag. The kit easily fits into the back of a bird vest, a fanny pack, or a back pack.
The only other thing one need consider before trekking the countryside is a resource, more valuable than gold, and under appreciated until demand is greater than supply; The bog roll. I think I'll let you draw your own conclusions on that subject.