I'm surprised no one has mentioned the "ten essentials:" matches and lighter, extra food, water, first aid, extra warm clothes, map and compass, multi-tool, light, raingear or tarp, sunglasses and sunscreen.
this is the list found in a lot of avalanche educational stuff. but even this seems like a lot to me. when i go out, i look at where i am going and think about the possibilities of getting stuck and go from there. think about having to get rescued. would it mean a few hours or a few days? i mean you don't need rain gear or a tarp if there is no chance of snow or rain, and you don't need a headlamp or a fire starter to duck the ropes for a lap. i try to go as light as possible so i can ride and hike with less which means your day can last longer and you get to ride more. usually in my day pack i have probe, shovel, water, sunglasses/goggles, board tool, knife, small first aid kit, trail mix (and possibly a sandwich), gps, map/compass, snow science kit, hiking hat/helmet, a few pieces of extra hardware, an extra fleece layer (i usually just have on my shell), camera, parachord. this would be a day hike only and i always keep more gear in the car for a possible overnight stay (usually parked at the end of sketchy fire roads) including a sleeping bag and more food/water. i would certainly advise against carrying a tent on a day hike, but a sleeping bag in the car isnt a bad idea. just evaluate your route. if you are new to the area then go with someone who knows it better so you will have a better idea. also learning to build a snow shelter is always a good idea. i have spent quite a few very comfortable nights in snow shelters that were built in the middle of nasty storms.