Voted for tarp and tent but If I look at all the times I've slept out over the last few years, I find that the tarp is taking over from the tent. I like its flexibility and space (as well as light weight). I don't have much experience of natural shelters as I usually camp when on a journey and I've never felt comfortable about turning up in someone's wood (or field, moor, mountain, whatever) and building something when I'm moving on asap in the morning. The tarp is also suitable for loads of environments, such as our upland areas, where natural building materials are either unavailable or require quarrying equipment. I have also used "found" natural shelters, e.g. caves.