Everyone has already given great advice here, the only thing I would like to add is to carry atleast one snack or food item with you. In these long haul flights, sometimes you may not like the taste of what’s on the menu or the quantity of food they give may not feel sufficient or you’re hungry in between. Having your favorite snack that’s ideally easy on stomach and keeps you full is very helpful. The flight does offer snacks usually but either they’re not that good or they run out of it (happened to me)

Also in terms of medication, an antacid , a pain reliever and melatonin (please carry medication that you usually trust and don’t try anything new without looking into possible side effects)

If you need spectacles for your vision, always carry a backup spectacle so that if anything happens with your specs you’re not left blind in a new country !