First, you and your child will be much more comfortable on a long flight if you get her her own seat and take her carseat. By strapping her into her carseat on the plane, you will be able to keep her in one place (not jumping up and down all over) during most of the flight. Take her out every once in a while for a walk around the aisle (bathroom stop, etc.).
For my kids, I tend to pack a couple of favorite books, coloring books, stickers, snacks (animal crackers, lollipops), a sack lunch (pbj sandwich, raisins, juice box), small toys (3-4 Hot Wheels cars, small plastic animals). Don't forget any "comfort" items like a favorite stuffed animal or blanket.
With two kids, DH usually takes one child and I usually take the other. That way, each kid has one parent to keep him/her entertained.