Noise-cancelling headphones! Sony WH-1000XM4 or Bose Quietcomfort models both work great for long haul flights and have long battery life. I love my AirPods Pro but they're not comfortable for 6+ hours.
I bought a pair of Athleta Brooklyn Pants (they have different cuts: wide leg, joggers, shorts, etc.) for a recent long flight and 9-day trip. They are such a perfect travel pant! They can easily be dressed up, but they're as comfortable as sweat pants. They're super lightweight (meaning they take up minimal space in your bag and also feel breezy on your body), moisture-wicking, and quick-dry for summer (which means I could wear them for a bike ride and still pop into a cafe and not worry about unsightly sweat marks. They're also wrinkle resistant, so you can stand up not show 8 hours of sitting on a plane or days of being crammed in a bag. Bonus that it's made of recycled polyester. You can search Athleta for "featherlight stretch," which is the fabric used in these pants and see more options. I liked these so much that I ended up buying the Skyline shorts as well, which I also loved.
Definitely second the comfy stretchy pants and pashmina type scarf! Shoes that are easy to slip on and off. Definitely get yourself a portable USB charger and have it charged up on the off chance you find yourself without access to an outlet!
Definitely download lots of content before you go, whether it's books, movies, articles etc. I still remember a 5 hour flight from ATL-SEA where I counted on there being wifi so I could work, but the plane's wifi stopped working... I did so much forced meditation :-)