For shelter I usually use a tarp or more recently a Pyramid Tarp but if I’m travelling very light I can use a poncho which is pitched in much the same way but can also replace a waterproof jacket. A waterproof, breathable “bivi” bag to cover the sleeping bag is a worthwhile addition, especially under a poncho which is smaller than most tarps.
campers just use a light quilt, arguing that the insulation of a sleeping bag is compressed under the sleeper anyway. While this may have some merit and is certainly lighter, I find a good sleeping bag is more versatile and definitely much easier to use in a bivi bag.
Because of that compression on the bottom of a sleeping bag it is essential to have some additional insulation under you while you sleep. In some places this can be natural insulation like
bracken but in keeping with the minimum impact approach some kind of sleeping mat is a better solution.