At some point, as has been illustrated, you cross the line from, essentially, emergency gear, to being prepared. If you go hiking, particularly overnighting, you normally carry the gear you need with you-shelter, clotrhing, sleeping gear, cooking gear-so, essential survival equipment is kept minimal, if at all.
However, if youre taking a dayhike, and DONT pack that, you are looking for items that are small enough to stuff into pockets, or carry in a small pouch. The basic premise is that they are just in case items-things to get you through a couple of days, tops.
Me, I work in layers; I have a neck lanyard with my RSK Mk V, a combo compass/whistle/matchcase, I also have a pouch that has a heatsheet, a fully stocked survival tin, a small FAK, a SAK with a saw (I dont recall the model), a Silva compass, and a collapsible 1L water container. I also always carry a 1L canteen with cup, an Esbit stove, my Seal Pup knife, and my Leatherman. This is all, surprising, lightweight (except the canteen, when full), and all fits nicely onto a belt. So, I am never separated from it.