Just seen this, and some interesting differences in people experience.
1) Keep an eye on the weather forecast in the area you're going to hunt, take the appropriate clothing, warm hat, gloves, but be prepared for it to be far colder, and wetter. You will be able to leave extra fleeces etc', in the trucks, between drives, if you know you'll not need them. Same for other things, you may, or not need. Gators are really useful, to help if it's really wet/snow, or through the brambles.
If your trip to Turkey (same place I went last year) is similar, we shot in the lowlands, where it was chilly, but not "cold", but also up in the hills, where there was a foot of snow, and you needed good, warm footwear.
2) There were no stands/seats in Turkey, and I would suggest some sort of stool a "must", unless you're prepared to stand for the whole day, and carry your rifle. Sit on a stool, rest rifle on your legs. There may be no trees, or anywhere clean/dry to rest your rifle, and the stool can double for that too.
3) Flask for a hot drink, bottle of water, even in the cold, you can get thirsty. Snacks of choice. Most people make a sandwich from the breakfast table. I'd suggest snacks you can open, and eat quietly !
4) A fixed blade knife is worth having, even if it's kept in the day bag. Very occasional dispatch, cutting a branch on a tree to use as a hook to hang your day bag, off the wet muddy ground, etc'.
5) Ammunition, we nearly always carry more than we'll ever shoot, but you never know ! Spare mag, if your rifle takes mags, loaded, ready to swap.
6) I favour kitchen roll for hanky, and toilet paper. Stronger, less susceptible to the damp, and some wet wipes in a zip bag, or the compressed dry hand towels that expand when you pour some water on them. Bin liners can be VERY useful, if you do find yourself in a high stand, and don't want to climb down during a drive !
7) Odds, and sods: hand warmers, pouch, or HotRox type, I always carry a head torch, been caught out a few times, walking out the woods in the dark. Something dayglo, Peltors, or similar electronic ear defenders, they are a great aid to hearing things coming. Batteries for whatever you carry, power-bank, and cables. Two way radios, are great, if you have, or a friend. Few Plasters, Cable ties, or something to make the big keiler you shoot.
A small cleaning kit, for the field, and back in the room. A pull through is good, but I favour having the multi section rod type too, for the field, as having slipped in the snow, on the way to my peg, and plugged my barrel, I had a nightmare finding a straight branch, to push the cr*p out

and that was my 9.3mm, anything smaller, and I doubt you'd be able to clear it.
Sticks, are one of those things you will very rarely use, but you can take them, check if your peg warrants them, and decide. Boar often do stop, and have a look around, but it would have to be a pretty long way, before I'd get on the sticks.
Spare glasses, contact lenses, and I'm sure I've forgotten things. The one thing I do remember, is whilst you can be lucky with some pegs, and may have a very short walk, if at all, some pegs can be a mission, up very steep, wet, muddy, snow covered hill/mountain sides, so you only want to carry what you're most likely to need. You'll have worked this out by day 4
