books are fine once you hit 6-7 years old, before then they'll get bored of them after 15-20mn since they can't read comfortably

colouring in / drawing great, but same thing, 15-20mn

construction games are great but they need to be carried about, they require space and there are parts you can lose

in places that aren't adapted for children (e.g. in an airplane or train), a tablet can be a life saver. you keep them busy/playing for an hour or so, then give them their digital crack for an hour, rinse and repeat. audio books are a good compromise