Ordinary plain plastic garbage bags have many uses when hiking and trekking.
1. Slit one down the side seams, and open it up for an impovised ground cloth.
2. If you're hiking and get caught without raingear, cut a hole for your head in the bottom or a garbage bag, and slits in the side seams near the bottom for your arms and wear it like a pullover.
3. Trash recepticle
4. Holds wet towels, cloths or bathing suits after a mid day swim.