I found that on long flights (5+ hours) with DD, having a few presents, wrapped in (old, mixed) wrapping paper were very useful to dish out at intervals of no shorter than an hour. Cheap things from Euroshop and bargain stores, so small toys, colouring book & pencils/crayons, small lego set etc. I used to take about 6-7 packages to use between outbound and return flights.
She would also take her favourite teddy, a couple of toys (mostly for the holidays rather than flight), a book for daytime and a bedtime storybook, and snacks - some healthy that we knew she'd eat (kids meals on US airlines can be odd - like BBQ sauce all over mashed potato making it un-eatable to most people!) and some treats.
DH would load up the iPad or bring the DVD player from the car, which
would give us a couple of hours each way. Even if the plane's own entertainment wasn;t great (US airlines also tended to have older planes with overhead shared TVs and no music on radio - US, Delta, American - we've tried all!).
And there is always a couple of chances to get up and go for a walk to the loo and look out the doors at the back for a few minutes, or lean over Mammy/Daddy to look out the windows too!! (If the DC are not in window seats for entertainment or containment purposes!!).