A large plastic bag or space blanket makes a good emergency shelter. Even better, bring two or three; they also make good sleeping pads, sleeping bags and emergency ponchos.
Waterproof matches or a reliable lighter will help you get a flame started, but having emergency fire starters--balls of dryer lint, candle wax and cardboard, for example--will help you turn that flame into a real fire.
A small, portable filter or chemical water treatment tablets will help you avoid the misery and weakness caused by water-borne illnesses such as Giardia.
You can't pack a week's worth of food, but a fat- and calorie-rich candy bar might help keep you warm through a cold night or fuel your efforts toward building an emergency shelter.
Think emergency first aid supplies useful in a crisis--a splint, triangle bandages, burn treatment packs and gauze, for example--not small bandage strips. Add an emergency whistle or mirror to help you summon help.
Add Zip ties, duct tape, cordage and a sturdy knife, and you can build almost anything or repair gear that's been ripped, broken or otherwise damaged.