Knife - A simple knife is the most useful tool you can carry-or better yet, small multi-tools.
Extra Clothing - However, no cotton. Cotton retains moisture and loses its ability to insulate; a very dangerous combination. Layer for insulation and carry raingear. Think "from my head to my feet" and have something dry and extra for each area of the body. A lightweight emergency shelter such as a tarp or space blanket is also advisable.
Matches - Waterproof matches kept in a watertight case and something on which to strike them.