My Mummy's Handbag is great. It's a kind of pop-up book shaped like a handbag with flaps and a removable (cardboard) laptop. My son loves it and it's pretty small so won't take up lots of space in hand luggage. We got ours on Amazon.
Reuseable stickers are good, a couple of books (choose ones with lots to look at rather than just a story), a few sweets to suck in case they have problems with the pressure (or a bottle if you're still using them). We also tried colouring books but he wanted me to do the colouring for him.
Manage post-holiday expectations by making it clear that anything out of the ordinary is a holiday/aeroplane treat, like sweets at 9am or whatever.
Any medication (including Calpol etc) can be taken in hand luggage so if you think you might need it, take it.
Pack a change of clothes including night clothes and any medicines or creams (for eczema etc) if you need them, in case they lose your luggage (I'm looking at you, Lufthansa). Also some kind of waterproof bag (even a large sandwich bag) in case you need somewhere to put barf/pee stained clothes until you get to your destination.
Pack snacks for flight but nothing that is easy to drop into small crevices(like sultanas) or makes too many crumbs.
Related:
Be mentally prepared for screaming all through flight (just in case), and if anyone complains, ask them if they would like to look after your child.
Don't feel embarrassed about letting them walk up and down the aisle (my 3yo goes off the visit the cabin crew). Most passengers are pretty friendly and tolerant so long as they don't get their seats kicked in.