A change of clothes for yourself in case you get anything icky on you. Yasin did a poop on a plane that leaked out of his nappy onto me once - gross!!
They have blankets on most flights so if they're not security blankies maybe you don't need them - i just put a wrap between yasin and the plane blankets.
If you're formula feeding you might want to have a think about how many bottles you'll need.
You can get some brands of formula in stick packs - its much easier than measuring although SIL has a special formula carrying thingy that has compartments and you put pre-measured forumla in them.