Walker
Prepare for a busy flight. I'm not going to sugar coat it for you: the period of early toddlerhood, from around 12 to 15 months, was hard for our family. My son was too old to want to sit still and sleep during flights (red eyes were out now), but too young to be enticed by the iPad for any length of time. I would prepare with lots of small activities, from books to toy airplanes to stickers and crayons, that captured his attention for short increments. Then, I'd pray he took at least one nap.
Runner
This period of dexterous mobility, from around 15 to 24 months, was an improvement from my son's earlier toddler days for one reason: he was suddenly interested in the iPad, and it was a game changer. We would load it with a variety of videos and age-appropriate games that could capture his attention for chunks at a time. But he wasn't yet mature enough to be enraptured by the iPad for the entire flight, so I would also bring along the aforementioned activities. The periods of iPad use took the pressure to entertain him off of me and made the flight go by faster.
Pro Tip: Trade off toddler duty with your partner. If my husband was with me, we would each do a 45-minute shift of being responsible for our little guy while the other relaxed, switching back and forth until landing. This way we each got a little time off to read or watch DirectTV and could come back to our next shift feeling refreshed.
