Mine have only got air-sick when there has been bad turbulence (apart from general throwing up while they were small babies), but you never know.
It's definitely a good idea to take a couple of changes of clothes for her, and at least one for you, just in case of sickness or spillages (fairly likely).
I wouldn't bother with earplugs, but DVD player or ipod with games and stories on, plus headphones she is comfortable with, are also a good idea.
Lots of small toys, stickers, snacks etc, brought out at regular intervals, help keep boredom at bay.
If you want to encourage her to sleep, try changing her into pyjamas at the right sort of time, and bring a snuggly blanket and soft toy. But be warned that they turn the lights on and off at odd times so everything may be lit up just when you want her to sleep.