One year when my kids were about that age, driving back from Orlando, we counted all of the McDonald's signs that we could see from the highway

That gave us something to look for and kept us plenty busy cause there are tons. You could also count something else, red cars, big trucks.
Make or buy car bingo games, you know, find a truck, a street light etc. and mark them off.
How about making up some "school" work pages, simple stuff for a 5 year old...like letters, numbers, simple adding, whatever level they are at.
Watch license plates as they go by and make a game with them, like find license plates with letters from your name and spell your name out, whatever you can think of to do with letters and numbers.
How about making up a story, each person takes a turn making a sentence or a word in the story. See what funny stuff you can come up with.
Ever seen the MadLib books. They are stories with blanks and you fill in the blanks before you read the story. Eg. they say name a noun, noun, verb, adjective etc. Then you go back and put them in the story and usually get funny results
Those a just a few ideas that come to mind quickly.
Have fun