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
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
Going to update my post from 6 years ago as I've changed a few things.
Still carry the normal stuff I listed in the first line. My fire kit remains pretty much unchanged- it's robust as getting a good fire going is really going to increase the comfort on an unexpected night out. My first aid kit is a little more robust to handle bleeding incidents better. I've added a Garmin inReach Mini and basically don't leave home without it. Satellite communication is one of things there really is no reason not to be without if spending time in remote areas.
One big change is I rarely hunt with a daypack any more; I've instead gone to carrying my frame (SG Krux) with a 50-ish liter bag. I can get one full load of meat out without hiking back to the truck. I tried smaller bags with my frame, but the 50 liter bag cinches down about as small as the smaller ones and isn't much heavier. It also allows me more volume to carry a few more things that would make a unexpected night out a little more tolerable.
I carry a Mountain Laurel Designs Spirit Apex quilt. It has a "poncho" hole so I can use it when glassing wearing it over the top of me if needed. I've found this to be great way to increase the time glassing in very cold weather. It also gives me another emergency insulating layer.
I carry a different bivy now. It's a custom bivy made my Nunatak, insulated with Apex and highly wind/water resistant.
I use a heavier (but higher r value) pad- a cut down Thermarest z pad (50" long)
I still carry a sil tarp, but a larger one. I found the 5x8' tarp a little too small when glassing in crappy conditions, the new one is 7x9' 12 oz vs 8 oz, but much better coverage.
Still carry a heavier insulated jacket, but also carry insulated pants now too (Apex insulated like the jacket).
A little more weight and a little more volume, but better prepped for a night out
