1. Snacks. All of the snacks. Stay away from sugary ones because there is no outlet for energy. I love the squeeze pouches because they have actual fruit and aren't a choking hazard. I also love those little packs of raisins for the older kids because it keeps them busy for a second.
2. Tiny new toys. Something like this, or a water thing with a book like this. It doesn't have to be anything crazy or expensive, just something to keep them occupied. As they get older...iPad. It makes entertaining them a breeze on the plane. We download movies, and apps that don't require wifi. It's a lifesaver.