My dad's two sheet tarp system for 4 people from the early 70's. Made out of swimming pool cover plastic it is very flexible as to how you can set it up...need to bring rope for the flexible part.

In the 60's and 70's (I did not keep track after that blush) , the tarp set up was the light weight set up. The other light options then were, a bivy sack, your poncho, and ala John Muir i.e. the clothes on your back and whatever you could find as a warm spot (I preferred a thick bed of pine needles myself). John Muir was gnarly and is the still the standard if you ask me. I have done all four "light" methods and ... well, let's just say I have definitely gotten soft and generally prefer the tent now (but still use the poncho method once in a while).