Since your kids are college age:
License Plate Poker - We start by everyone putting their ante in the pot. Everyone picks a number, 1-10 (or whatever). You count the vehicles you pass or get passed by. When you come to your chosen number, that license plate makes up your hand - A=Ace, J=Jack, Q=Queen, K=King, number 1 - 10 are same value. Whoever has the best hand after everyone has been "dealt" their "cards" wins the pot. We have played with quarters to use for video games/vending machines, wrapped candy, pennies (the winner picks the meal stop), etc.
Twenty Points - everyone picks a number 0 - 9. Each time they see their number on a license plate they get a point. Three points if it appears twice, and seven if it appears three times. First one to twenty wins.
Meet Your Make - select a number 1-25 (or whatever) as you pass or are passed by vehicles you count them. When you get to your number, that is your vehicle. Pretty good for a laugh now and then.
Cargo Count - take turns guessing how many passengers will be in the next car you pass or passes you. Correct guesses earn as many points as passengers. Whoever has the most points after a designated time or destination is the winner.
License Plate Lingo - Make words out of the letters on license plates. The goal is to make a saying that makes sense. For example - 219OTL could be Out To Lunch.
Radio Tac Toe - Select a radio station (We like to use an oldies station to give mom and dad a better chance.) Each player makes a tic tac toe grid and writes the name of 9 different songs. As the radio plays songs, you cross off the titles if you listed them. First player to get a tic tac toe wins.
Have a great trip and enjoy the drive!