I had a ton of busy bags last time I flew with the kids. I had a 10month old and 3 year old, with connecting flights and long layovers...on my own. Phew that was a long trip.
Some of our busy bags -
Large beads and pipe cleaners. (I used large foam beads)
Felt busy boards - make a cupcake and make a monster.
Lacing boards from M&D brand
Color sorting tray
Small puzzles (wooden, from dollar store)
Small bag of cars and simple felt "road map"
Felt sandwich making kit.
On the felt busy boards - I cut a piece of cardboard the size to fit in our baggies, and then covered it in felt. I then cut out all the pieces for each kit (look on Pinterest for pattern ideas). The pieces then stick to the felt board as the build the monster/cupcake/etc. And it all fits back in the bag.
The car/map bag - I got a piece of green felt and sewed a simple road shape on to it from grey felt. It can be folded up to put in the bag.
Other ideas - felt doll dress-ups on the same boards, cakes instead of cupcakes, making scenes (castle, forest, ocean).
It took a week or two of working on these after bed time but it was SO WORTH IT, not only for that trip, but for any time after. I can pull these out at home or away and get 5 minutes to myself or with limited scripting from my little guy.