Things that she can't make a big mess with!
What is she into? I can't remember back that far.. we went to Aussie with Ella at that age.. I think I took some washable stamps from the Warehouse.. she loved those.. she stamped all over the notepaper I took with me. Buy her a new book too if she's into them.. some little snacks for her to eat. It's not really that long on the plane, if she hasn't been on one before, the excitement of it might just be enough!
My one piece of advice: if you take a drink bottle for her with a straw, open the straw up first before you take off.. otherwise the pressure when you're flying will mean that when you open the bottle, your toddler will be wet. Was kinda funny when Ella did it, but not so fun having a sopping wet child to change