Ah, that really is the question! I did try to restrain myself from both lessons of past experience and I did want to let them each pick something there. Aside from the lesson of my kids aren't into pin trading (doh!), the main lesson is that too much stuff is just TOO.MUCH. They tend to pass over things especially at that age. So I would also think multi-use things -- clothes, practical items, stuff for the plane and so on. I definitely did not do all this but here are some ideas:
* books, coloring books, crayons -- easy thing to have for the plane
* stickers -- relatively disposable so fun for a bit
* swim stuff -- beach towel (can use as a blanket on the plane if you like), flip flops, swimsuit, sunglasses, hat
* clothes -- can't go wrong here. Even includes stuff like princess dresses and pirate or star wars get up.
* movies -- plane entertainment AND home entertainment. Win-win.
* music -- Disney music always a fav
* for those two, kidzgear headphones for the plane if you don't have them. Very comfy and kids love them.
* if girls, jewelry -- just some of those beaded stretch bracelets can make them pretty happy
* practical -- food items like fruit gummies in Disney brand, Disney bandaids to take with you into the parks, disney water bottle, disney socks
* toys! -- little action figures and/or a plush