1. Utility cord. Also known as parachute cord, paracord, or p-cord, this essential and inexpensive item typically comes in 50-foot lengths and runs 3 millimeters in diameter. Its uses are almost endless: shoelaces, tent and tarp guylines, clothesline, food hanging, gear lashing, dog leashing, and the list goes on.
2. Duct tape. Create or buy a small roll for your repair kit, or wrap some around your water bottle or trekking poles for easy access. Works great as an emergency patch for your pack, tent, rain jacket, or other gear, as well as serving a million more potential uses.
3. Zip-lock bags. Choose thicker, more durable freezer-style bags that pinch closed. (Avoid sliders, which break easily.) The uses are myriad: trash bag, emergency water container, gear sorter and protector, waterproof map cover, stanky socks quarantine unit, and so on.