Lots already covered.
Make sure each DC has their own blanket and a favourite soft toy as well as choosing something to play with en route.
Paper and crayons (less messy than felt tips, no need to pare like pencils) - for car games, Xs and Os, drawing, etc. A large (a4 - not necessarily thick) hardback story book can be used as a table on their knees and then bedtime stories on holidays.
Good snacks include raisins, plain (I.e. not chocolate!) biscuits, apple chunks (I core and cut into wedges, then pop into plastic tubs or ziplock bags), fun sized haribo jellies etc. And each having their own is good.
And a drink bottle each - DH may need a larger bottle in the front to refill if necessary.
Bin bag for rubbish. But we also (as dd was prone to travel sickness) carried a plastic tub to throw things into bit that could double as a sick bowl if needed. (We also have lots of air sick bags from travel, we used to collect them for car journeys).
Also, always had a towel, pocket pack of tissues, change for dd and clean top for me in an easy to access bag (as I was usually the one covered in sick or whatever was flying). I usually kept a few nappy sacks in there too to put anything that needed a wash or otherwise contained until we could stop properly.
Apart from I spy etc, another game I remember from childhood and sometimes dd is to guess how long a mile is as you are driving.