There are lots of variations of this game but this is the one we play.
We use whiteboards for many of our games (as a table, scoreboard, drawing sheet etc) and we will draw 6 or so items on the board.
Each time an item is spotted it gets crossed off, the first person to see all six items is the winner. The nice thing is for some journeys you may know of landmarks or particular items that will definitely appear but you can also give slightly harder or more items to more experienced players.
A non-competitive variation is to get them to keep a tally of each item and then compare at the end of a trip. eg Cows – 8, Red Cars – 3, Campervan – 5 etc