Something so simple, inexpensive but essential for any survival pack: Paracord
Paracord is easy to carry with you and really helpful in many survival situations for building things, first aid, climbing, repairing and a roll of paracord is under $10, light and easy. Don't forget your paracord. Or if you don't want to carry it let the dog carry it:
There are numerous ways to use PARACORD in a survival situation. Here are but a few ideas:
First Aid: Paracord is useful for Sutures, Splints or a Slings and also combine with a couple of poles to make a stretcher to carry a friend.
Sourcing and Keeping Food: Fishing line or even a fishing net. Trotlines with baited hooks can be left in the river at strategic points. Also Snares or Traps are essential and once you have food you will need to Hang Food. Paracord can do all this.
Security: Self defence Paracord techniques include a simple slingshot, also good for tripwires and trip alerts around your camp. A Monkeyfist could be a very simple weapon to create. Think of a medieval ball and chain on a smaller scale. A rock or a larger ball bearing could be the weight. A pool ball is a good item to practice creating a Monkey Fist with. Another idea though: What if you need handcuffs?
Travel and logistics: Imagine the tree house scenario. What better way to get up and down the tree than a rope ladder that you can just pull up behind you. Paracord is ideal for heavy rope making of all kinds. Also building shelters and securing tents and tarps in a storm. Building a raft or an aid to cross rivers. Tying the team together can assist as long as the majority of the team stays on firm ground at all times. No point if everyone gets swept away by the current.
The list goes on: hauling timber or attaching extra gear to your pack, the uses of Paracord are endless. The real issue is how much have you practiced? How good are you at knots. I would definitely recommend putting Paracord in your survival kit but also you need to know how to make at least the basic types of knots and braids to make Paracord really useful. As always preparation is everything!