Books are always good, so are magazines (SpongeBob and My Luittle Pony are my girls' favorites). Bring his/her favorite blankie and/or teddy or other lovey, several pacifiers if she uses one, as they can fall and it's hard to rcover it between/under plane seats while you are trying to sooth your toddler or put her down for a nap.
For foods, I would bring:
- small packets of crackers (there are organic cheddar bunnies at Whole Foods, they come into a box with 6 packets)
- cubed of cheese of your toddler likes it
- apple slices, carrot sticks, cucumber slices cut up, soaked in water with lemon to keep them from going brown and then packed in those small ziploc containers with lids
- water or baby tea
- pre-mixed formula (if applicable)
Also bring some Bach Rescue Remedy in case she gets agitated you can put some on her wrists, and a mister with lavender essential oil mixed with distilled water also for relaxation (shake before use and spray some on the blankie or around before the nap).
Select a few of her favorite toys, and make sure you bring a carrier that's easy and quick to üput on and comfortable for both of you so you can easily carry her and the hand luggage when you don't have the stroller.
I hope this helps - have a great trip!