Wear:
Compression socks
Comfortable clothes that you can sleep in, in layers (like a vest top, a long sleeved top, a cardigan and a scarf wrapped around your neck that can double as a head cover or a blanket as necessary, or just as a throw around your shoulders).
Buy a bottle of water after security.
Most airports have water fountains somewhere to fill your own bottles if you bring them through security empty. (I now take my reusable coffee cup also, as it keeps much warmer than the takeaway cups do, mine doesn't leak so that helps).
Have some chewy or boiled sweets. If you want, bring a few snacks, but not loads - you can kill time in the airports roaming for them, and they will feed you on those long flights.
Bring a lip balm in your "liquids for carry on" clear plastic bag. I usually have my toothbrush, toothpaste, deodorant, cleanser, moisturiser, comb or small hairbrush, emery board (nail file) in there too and sometimes makeup if I feel I will want it.
Decent book that you are enjoying, and probably buy a magazine at the airport.
I often buy the books of sudoku puzzles, but rather than bringing the whole book, I rip out a few from each section (easy, medium, hard) and bring those, along with a notepad, pencil, pen. (I don't care if I mess them up and don't look at the solutions anyway - but once I've tried the ones each side of a page, I can throw away the page, and it's much easier to have a couple of pieces of paper and a pencil tucked into a pocket than the whole book).
Headphones, phone, spare power bank and charging cables for phone and powerbank.
Ipad or similar is very useful - games, podcasts, downloaded tv or movies, downloaded books etc.
And I usually bring spare socks and spare underwear - it's nice to freshen up with clean ones at the end of an overnight flight, and also its ALWAYS useful to have a spare pair in case luggage doesn't arrive to get you through until you can find shops etc. (A little washbag pouch is sufficient for those latter 2).
But my carryon bag is quite small - 25l backpack is plenty and has room for any shopping as well.