Jeans, a comfortable pull-over shirt, nice thick socks, and comfortable sneakers.
If you are in business class they will give you ear-plugs and an eye mask. If not buy your own to help you sleep.
I flew Turkish Air last year between Israel and NY and all the passengers got these for free.
Best thing to do is get rip roaring drunk on the free drinks.
I don’t think any airline serves alcohol that’s free anymore.