We play the license plate game - see who can find the most different license plates. Might be hard with a 3 and 5 year old.
The other game we'll do is go through the alphabet finding words that start with that letter on a sign, other vehicle or building, etc. (e.g. I get "A" for a sign for Albany, now everyone looks for "B" words). Again, might be a bit above grade for your kids.
We've spent time doing tongue twisters. My daughter recently fell in love with "99 bottles of beer on the wall" (much to my chagrin).
Books are big ones for us- picture books and activity books (crayons and pencils).
Many things that they do at home can be done in the car. If they play with Barbies or GI Joes, they can play with them in the car.
There's also "watch the world go by".
Sometimes my daughter will count how many people she can get to wave at her.
Putting music on and singing.