Along with the "normal" bits- robust fire kit, first aid, repair stuff, good headlamp/spare batteries, nav items (compass, map, gps)- the stuff that's going to really help me get through an unplanned night out- are:

extra clothing- a good insulating jacket (w/ hood!)- temp/weather dependent on which one, but I like synthetic due to it's ability to handle wet better (than down), particularly Polartec Alpha and Climashield; a fleece balaclava; fleece mitts; spare wool socks and a good shell jacket (again hooded)

sleep system- this won't make for an overly comfortable night in really cold conditions, but will keep me alive smile

diy climashield quilt- climashield 3.6, quilt weight 15-ish oz
SOL bivy- this is their newer one and actually breathes 8-ish oz
ccf pad scored to fold up (like a z-pad) 4-ish oz
5x8 sil tarp 8-ish oz

relatively light and very low volume, easily all fit into a 20-ish liter daypack

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