We should all know the 10 essential hiking items:
1) Map/Compass
2) Hydration (water)
3) Food
4) Rain gear/extra insulation
5) Fire starter
6) First aid
7) Knife
8) Flashlight
9) Sun protection
10) Shelter
1) If you know the area well, map and compass can be optional.
2) Water might not be in liquid form, so having a way to melt snow and ice is a good idea, such as a metal cup.
3) Food - the kind you can eat frozen. A frozen candy bar can break teeth! I like candy orange "slices" frozen to 20 below though. Since you have the metal cup and a way to make fire (see below) some hot chocolate or tea bags would be good to have too.
4) Rain gear/insulation You most likely at least have a shell of some kind and a warm layer to put on. Have a piece of closed cell foam at least big enough to sit on. A spare set of base layers and a change of socks is a good idea too. The base layers you hike in are going to get wet or damp from sweat. If you have to stop and spend the night, having dry base layers and socks to change into could make all the difference in the world. You might even want a spare pair of gloves and hat.
5) Fire starter - and not just a lighter. If you need to make a fire, you need to make it in hurry and have it start right off. Carry fire ribbon or Vaseline soaked cotton balls or a few esbits cubes. You need a way to make that hot cup of tea or chocolate. A beef stew can with some holes punched into it can make a good stove/pot stand for wood or esbit cubes.
6) First aid - I consider this optional. Not a lot you can carry which would be helpful if your really in need.
7) Knife - a good one you can use to make fire wood with.
8) Flashlight - a no brainer here. Spare batteries too if you can't remember the last time they were changed.
9) Sun protection - also optional depending...
10) A basic bivy sack or even a space blanket could get you through the night. One of those emergency Sea to Summit sacks aren't very expensive. Maybe a bag liner too. Not as good as a full sleeping bag, but enough to keep you alive through the night.
Of course, if I'm just going out to walk around in the woods for a few hours around here, not likely I'll carry any of the above, but if I'm heading up towards tree line, maybe.