How do you keep a toddler busy during a flight so she won't run back and forth to and from the lavatory?

Before we fly, I got a habit to tell my children what will happen on the plane and how I want them to behave. I will pack some snacks, books and toys for them. All these help to keep them occupied at least for a little while. Take these items out one at a time to maximise the length of time they will be happily occupied.