We have just got back from a 2 week holiday in Seattle USA with our 22 month old and wanted to share some of our experiences.
Firstly the flight- if at all possible book a seat for them. Most airlines don't expect you to book one so you have to do it on the phone rather than online but it really is worth the money. We thought we had booked one on the outgoing journey only to find out that infact we didn't have one. (Needless to say we booked one for the return journey)
HAve lots of different activities for them. I had put everything in small ziplock bags so that you can get things out easily. We packed lots of small books, cars, threading beads, craft materials and even glue ( I got one less that 100ml) and we even made puppets out of the flight socks. Aquadraw make portable sheets which were great and we also packed the portable DVD player which was a real godsend at some points when we needed a bit of down time.
Take lots of small snacks with in ziplocks so that you can access them. I also carried on a small cool bag with several different meal options. This was great as we could feed our son whenever.