A car is a hostile environment for most children's things. No table, no floor, limited light, and a proportion of children who cannot look down at a page without regretting it within twenty minutes.
So the usual advice, take a book and some puzzles, fails on contact. Here is what actually survives.
The constraint nobody mentions
Anything that needs looking down makes a lot of children sick. That rules out most books, most drawing, and every puzzle.
Which means the good car activities are either listening, or looking out of the window, or talking. That is the whole category, and once you accept it the choices get much easier.
Listening
Songs they can join. The single best car activity there is. A song a child already knows turns four hours into a concert, and a child singing is a child not asking how long is left.
Audio stories. For seven and up, mostly. Under that, attention wanders without a picture.
A story told badly by an adult. Free, and better than any of it. Start a story, hand it to a child, take it back when they run out.
Looking
Spotting games. Traditional, endlessly modifiable, needs nothing. Count red cars, find the letters of the alphabet on signs in order, first to see a bridge.
A camera, even a very cheap one. It converts a boring window into a task.
Talking
Would you rather. Free, absurd, and it lasts longer than anything you could buy.
Twenty questions. The best game in the world for a seven-year-old and the first one where they beat you.
The thing that ruins a car journey
Not boredom. Hunger, and the argument about when to stop.
Every experienced parent knows this and every list of car activities ignores it. A child who is genuinely absorbed in a game will still fall apart at hour three if nobody has eaten, and no present fixes that.
Feed them before the fun runs out, not after. It buys back an hour of everything else on this page.
What actually to buy, if you want an object
A card game small enough for a lap. Works at services and in a hotel, if not while moving.
A magnetic travel game. The pieces stay put, which is the whole point.
A cheap camera or a torch. Both convert the environment rather than competing with it.
Where mine fits, and where it does not
Honestly: the storybook is not a car book. Four verses of rhyme read by an adult in a moving car is a request for trouble, and I am not going to pretend otherwise.
What does work is the twenty-six songs. They are downloads, so they are on your phone rather than needing a signal, and a child who has heard a fable at home will sing it in a car for a hundred miles. That is the actual use case, and it is the one that made me put songs in the set at all.
The card deck works at the services, at the hotel, and in a queue. Not while moving.
$49.95 during the launch, and the songs come with it rather than as a subscription, which matters when you are somewhere with no signal.
Before a long trip, the cheapest preparation is free: print a few colouring posters for the hotel, not the car. Ten things to do with a deck covers the rest.