I flew with my son the first time when he was 17 months, and the second flight he was 18 months and we flew overseas! So, that said, here are my suggestions:
- New small toys that he's never seen before (the dollar section at Target is great for this); pull them out one at a time so each one lasts a while. Keep all toys in a zip loc baggie, so they are easy to find when you need one.
- A magna doodle as others have mentioned; little wind up toys that can "walk" on the tray tables; little cars, little toy cell phones.
- Parents brand of toys sold at Target makes a really cute item called a MyPod that is entertaining and plays cute music. We bought the version with classical music, so it wouldn't bother other passengers. It is around $15 I believe.
- Books with lots of flaps.
- It is important for your son to swallow on the ascent and descent. Individually wrapped snacks worked best for me -- gold fish, mini strawberry newtons, etc. -- as well as pretzel rods. Keep all snacks together in a big zip loc baggie so they are easy to find when you need them.
- My son got bored with his sippy cup, but he loved drinking out of the bottled water that I bought at the airport. That was a lifesaver! I also bought a drinkable yogurt at the airport and put it in a sippy cup with a straw, which worked pretty well too.
- Check as much as you can so that you are only carrying the things you really need -- diapers, toys, snacks, phone and wallet. It will make things much easier.
Good luck!