We fly in a couple of weeks with my almost 3yo and I've bought wipe clean activity books and a set with cards, fidget toys and stuff.
My eldest has been obsessed with numberblocks (not these, the math link cubes) since he was three and he's now five. I wouldn't even take those on a flight though as there are so many pieces to lose. On a flight I'd take sticker books, story books, drawing, maybe a small amount of duplo or some cars/trains if they love those. I'd also indulge in screen time and kids podcasts/music if I'm honest.
https://a.co/d/3snjPPV