We have flown a few times with our 2 children including when ds1 was 21mths.He is now 2 and we are preparing for a long trip NZ-UK in a months time so are stocking up on things.
He loves the self inking stamps so we have a couple of small animal ones that don't make too much mess and can be stamped on hands/feet and paper (if we're lucky!). Selotape is good and seems to keep him busy for quite a while. Pipe Cleaners (coloured) I have been told work well as they can be bent and made into all sorts of things.
Stickers seem to work for our little one when he sticks them on his Daddy's nose/ears/hands etc. rather than in books!!!!!! I guess he is learning body parts this way too!!!!!
I have done multiple long haul flights with my 2 ds 4 and 2. I would look into the crayola markers the ones that only write on the colourwonder paper. Small toys my ds loves plastic creepy crawlies (not the really small ones as he might swallow it) I also carry a portable dvd player(not cheap sorry) don't take everythink out at one one at a time as you need them. My lo loves book with flaps etc too. Snacks like small crackers and chocolate buttons etc may also keep him occupied for a while. I also use small play pots and spoons for my ds to pretend to cook and the veg which are in two halves joined together by velcro wich he pretends to cut and cook in the pot (sorry if that does not make sense) Try to make sure he is tired but not over tired so that he is likely to sleep for a few hours on the flight too. I hope this helps.
lego, stickle bricks, playmobil - ds loved rhe big versions of these on journeys - just take a small selection e.g. lego car, driver and 3 bricks. small pot of playdoh. play phone (they are usually quiet to protect ears so shouldn't annoy other passengers!) raisins, bananas, pot of grapes etc. Have fun!