Always be prepared for an emergency. A good question to ask is, "Can I get back safely with the equipment I’m taking?" Here are some items to consider:

Extra key (One could get lost in the snow).
First aid kit.
Pocket knife (some carry an ax).
50 feet of ¼ inch nylon rope.
Map, reliable compass.
Water proof matches.
Candy bars or other high energy food (or a one-day supply of food).
A pair of snowshoes.
Flashlight.
Flares.
Emergency shelter.
Emergency shelter material (tube tent, tarp).
Space blanket.
Sun-blocking cream.

For convenience, you might like to consider some luxury items if you have room to carry them.