Hi L.,
I have 6 kids and always find that flying can be challenging! Lots and lots of high energy individual snacks and tons of drinks..when you think you have enough..pack more! Add some treats too for when you need a little bribery...lol!
As far as activities...go to the nearest dollar store! Cut paper into shapes at home and then give her stickers to decorate them. Cut out pictures in magazines (or of family you are going to see) and bring glue sticks. Workbooks are fun. A couple of small dolls and toys she hasn't seen before. A new puzzle or two. A collection of new plastic toy animals..the trick is lots and lots of new stuff. None of it needs to be expensive but being able to switch activities every few minutes is the lifesaver. Punch holes in paper plates and let her string a shoelace through it. Hit the crayola aisle and get some of those markers that only work on special paper...fascinating and no mess. Playdough and cookie cutters. New sticker story books. Card games. Colorforms. (remember don't pack any scissors not even kid ones!)...
hope this helps.
L.