We had a long haul flight to get back from Stockholm to Hong Kong, which is now a direct flight (Tjoho!). That cut about 10 hours off our journey from when we started going back-and-forth a year ago. Now we have 15-month-old, so it makes things more interesting.
He was surprisingly good at staying in the spots where our seats were, and didn’t insist on going walking around like he did on other flights. He instead wanted to either read, sleep, or sit on the floor most of the time and simply play.
There is not much you can do differently on a flight with a 15-month-old than if you were at home with them. They need to be stimulated, they need food at the right timing, they need snacks, and books and toys. In other words, you just need to treat them with respect for their routines, and things will hopefully go smoothly for all parties.