We flew with mini to Hawaii when he was about 10 months old - about 11 hours flight. We didn't buy an extra seat, but reserved the seats with the bassinet in front. Highly recommend to do that even with the extra seat. Mini was still small enough to fit, just barely. We had an overnight flight which was great. Mini slept in the bassinet most of the way. On the way back was a daytime flight. Got the bassinet seat again, but this time mini had no desire to sleep, but the extra floor space was still very useful for him to sit and play.
Other than that, snacks, books, new toys - preferably ones that don't make noise.
Extra diapers and diaper bags, portable changing mats. Lots of extra clothes, and a set for you.
Extra bottles and formula if she's into that...
A baby carrier without metal in it - useful to get around the airport, and if she's asleep you can wear her through security. (unless you forget the littleperson is holding a toy car in their fist that sets off the alarm...
Oh, and don't forget her blankie on the plane.
That's all I can think of right now. At 10 months he was actually pretty easy. Loved playing with all the latches and window shades, and the emergency info card was the most fascinating read ever.