How old are your kids?
An alphabet game...find the letters of the alphabet (in order) on signs, license plates, etc. Generally we play it as a cooperative game but limit the number of letters each person can find in a row so that everyone has a chance to add letters.
"Would you rather..." game...someone starts with two choices "Would you rather be able to fly or swim without having to come up for air?" Everyone in the car answers and the next person picks something. You can theme the game or just go with whatever.
Rhyming or starts with...great for younger children, though it gets a bit tedious for older children and adults. Pick a word and go around the car rhyming or coming up with another word that starts with the same letter.
One of the many variations of "I went on a trip/picnic/to the beach/to Disney World/etc." Starting with the letter "A" and working through the alphabet, select something that starts with the letter and fits the theme. The next person must say the list of things that came before and then add an item for whichever letter is next. (I'm not explaining this well...) For example:
Person 1: I'm going on a picnic and taking an apple.
Person 2: I'm going on a picnic and taking an apple and a banana.
Person 3: I'm going on a picnic and taking an apple, a banana, and my car.
Pick a color and type of vehicle to try to spot.
Make up BINGO type cards with things that you expect to see along the way. Use some of the Dollar Store toys as prizes.
Let the kids spend some time reading the map and charting where you are going.
Books on tape from the library are wonderful!
Hold back some toys for later in the trip, so that there is something new to do.
I'll keep thinking.
Beth