Okay, don't think badly of me, but we gave our DD who was three and a half, a dose of children's gravol. Not only did it prevent any type of air sickness but it put her out for an hour, at least, both on the flight there and back. She even nodded off of the take off during our return home!!!
I would also try to explain how a plane trip works, show her the seat take off position by sitting her in a chair and then lifting it gently backward ( you know what I mean). We did this with our DD and she was excited to be going on a "ride". I would also talk about the importance of the keeping her seat belt on and being relatively quiet. I did not do such a good job explaining this to our DD, I never really thought of it. She did not want to keep her seat belt on on our way down and she got a bit loud. She even kicked at the seat in front of us. I was mortified. It got worse though. When she would not keep her belt on the stewardess came over and gave her and us royal he!! It was embarrasing and so frustrating. After we got to the hotel and had all relaxed a bit DD was able to tell us that she got scared of the loud noise and just wanted to look around. I can totally understand her curiosity and her fear and next time I will be better prepared. On the way home she was much better. No fuss over the seat belt or anything.
Oh, if you have a game boy you could take that. Much easier to tote around and some of the kids games are fun for a small child to mess with. I do not normally give my DD the game boy (it is her dad's) but for this trip I made the exception and it really helped to keep her distracted.
We gave our DD the fruit chews to eat to help with any ear popping. She was fine there and back, but it was good to have them. Oh, with the size limits on what you can bring on the plane there is little choice for drinks. But, I was able to take three or four juice junior drink boxes. They are about two thirds the size of a regular drink box and they come with the straw. You can get them at A & P. I was allowed to take these on the plane in my carry on and this was just last December.
Oh, another idea is to take some nail polish ( you can get kid's kind in small containers) and the paint her nails, toes and all!!! This will kill a few minutes and she will love it!!!
Have a super time!!!