I spy. Puzzle books, colouring in books, reading books. Card games are good, so many different games you can play with one pack of cards

Pack a backpack with colouring, playing cards (UNO is a great game!) books etc. Also talk to the airline stewards. They can help as many airlines have kids activity packs on board. Also try to encourage sleeping by buying a fun neck pillow and eye cover. Depending on the child’s age you can get special inflatable pillows that allow them to lie flat on the airline seat. I think one brand is Plane Pal but I’m not 100% sure.
Good luck!

Surprise the child with a new toy to occupy themselves with. Books are great and keep plenty of snacks on hand.

You can get up and do regular walks up the aisles. You can make a game out of it by getting them to look for someone in a red shirt and then someone wearing yellow etc. Otherwise it’s unfortunately mostly sitting activities like reading, coloring etc.