I'm afraid I go for noisy electronic toys that make diverse noises, flash lights and sing songs to them. Whip this out when they are bored with everything else (they should ideally be some things they have never played with before). May slightly annoy some passengers, but far better than constant whinging or screaming.
the most entertaining thing will be the plane itself ... endless fun opening and closing the fold-down tray, playing peepo with the unwilling passenger behind, exploring every inch of the passenger loo, hand basin, soap dispenser and fold-down changing thingy, and of course the obligatory walking up and down the aisle regardless of the steward's attempts to progress with the trolley. In-flight entertainment may enthrall if sufficiently brightly coloured, even if no sound.
Every flight I have taken with ds, he has looked at the array of toys and books for all of 5 mins apiece, said the toddler equivalent of "meh" and returned to the excitements of the window blind. The exception being stacking cups. Brilliant. For extra diversion ask staff for some ice - ice cubes in a cup are fantastic fun.
Whatever you do don't take anything with small or multiple pieces as you will spend the entire flight groping around your fellow passengers' feet to retrieve the pieces.
The winners for DD's (3 and 1) were magic slate things, calculators from pound shop that beep when you press a button and shaped like mobiles and stickers with pads to put them in. Anything with small bits will fall on the floor. It's the law. We also took a DVD player with some night garden dvd's which was a godsend.
straws - esp those with bendy bits to curl over lip of cup. they can chew them, bend, batter and all with no noise - also they take up next to no space, and it doesn't matter if you lose them. You can join them together and make shapes too to entertain them. I always had one with me when DS was little no matter where we were.