What we've learned from our travels:
1. No car seat if it's an overnight flight. Our daughter's first overnight, to Scotland, the man in front of her reclined his seat and our 26 month-old starting yelling "my legs, my legs are trapped." The man would not move up his seat and her carseat was rendered useless. Our daughter sleeps better sprawled between the two of us, head on mommy and feet on poor daddy. It breaks my heart, as a former USDOT employee, to say dump the carseat, but heck, if the seats are too close together, then that's that.
2. Lollipops for the ears. Pass one out taking off and one landing. No ear pressure problems. Since our kid doesn't get much candy at home, getting a lollipop while flying makes airplane rides extra special.
3. Small/tiny backpack with lots of pockets. We bought a small Kippling backpack, just her size. I only bring it out for airplane travel, loaded with stuff from the Dollar Store.
4. Dollar Store! Favorite items: Pipecleaners (to twist into bracelets, animals, etc.), card games, beads for stringing, toys, glow bracelets (for evening flights), bendy animals, digital watch w/ stopwatch. I once bought a mini "aquarium" to fill with water and watch the "fish" grow. Big hit, my daughter monitored it all the way to Australia.
5. Sticker book. Bought a mega one on Amazon and only bring it out on plane rides.
6. Change of clothes and plastic bags. Also jammies for night time flights. Tooth brush, too, for night time routine.
7. Empty sippy cup. Fill with water later.
8. Stuff inside of stuff. A necklace inside of a pretty cloth bag inside of a small box.
9. Travel tissues. Our daughter loves to open packages of travel tissues and cover her doll, make a tablecloth, or make a toddler attempt at creating "art." Weird, but it works.
10. Stuff for drawing. One or two coloring books, notebook (sometimes with graph paper), two sets of crayons (in case one gets lost).
11. Small doll with long hair. Bring also small hair baubles, combs, etc. for "stylin'" as my daughter likes to say.
12. Books.
13. Snacks.
14. Wipes. Last time my daughter was grossed out by her tray table and wanted to clean it. Other times we've needed to clean our daughter.
Seems like a lot of stuff above, but if you bring small of everything, it isn't much. For instance, we bring only thin paperback books. There's never "too much" stuff to entertain a kid. Luckily, Qantas has personal entertainment systems, which worked like a charm. When my daughter woke, she would watch a little bit of Bindi or The Wiggles, then fall back asleep.