Don't get too much. You'll just end up lugging it around, and your hair will go static as you constantly have to root around under the tray/chair to retrieve when she drops...
Do get a small travel bag or backpack that has her toys in it, otherwise you're rooting round in your bag, along with everything else that's in there, looking for things.
Check if your airline has back of the seat tv's. Practise using headphones before you go and stick the cartoon channel on if they do.
My ds was lulled to sleep for a great part of our journey to NZ, via USA by the droning of the plane.
Make sure you have snuggly type toy/blanket for flight.
What toys does she like generally? Will she sit and play with something like a my little pony or is she more active? I recommend stickers, some kind of colouring if she's happy to sit and use felt pens/pencils. Also a small pot of fruity lip salve will keep her busy and you can distract with it every so often (ds liked the jellybean flavoured ones).
Ds also liked choosing a special packet of sweets for the trip - the travel tim kind. It also helped with take off and landing to have him eating a sweet for his ears.
Take an empty drink bottle/lidded cup and fill it with water/juice when you get past the security checks. You will be able to buy it in the shops there. Ask the steward to fill it up when they come round with the trolley so you're not having to wait to get a drink later. If you do put fruit juice in it remember that the sniffer dogs may be attracted to it when you land. They put ds's drinks bottle through the xray when it had ribena in it, so I would suggest a clear cup may be a better option.
Get the 3 seats by a window. Put dd by the window so she can look at the clouds etc and also lean against it to sleep. Then you won't have to disturb her if you want to get up while she is asleep. And you can easily lift her over you to get out with whoever is on the aisle rather than having to unbuckle, remove headphones etc.
Do take her for a wander up and down every so often, but try and avoid when the trollye is going up and down!