Here are a few of the games we like to play on road trips. Hopefully they can bring some fun and fellowship to you and your family/friends. While I didn't come up with them, I believe just offering suggestions and making others aware is truly beneficial.
This is my first instructable, so please offer any (constructive) criticism so any more I make can be more useful.

Also, please vote for me in the "Are we there yet" Challenge.

1. License Plate Game
2. 20 Questions
3. Famous Quotes
4. Catch Phrase

Step 1: License Plate Game

This game is so simple, yet fun for the kids (and keeps them busy).
The goal is to spot as many license plates from as many states as possible. They can be on any type of vehicle. From cars to trucks, or boats to trailers. While we usually work as a team to find as many as we can, you could also play a variation where you keep track of who spotted the states and keep track of points. The competition or the introduction of a prize may raise the interest level of stubborn passengers (we all know them or are one).

I attached a sheet to help with this game. If you keep track of points, you could simply write initials next to the state or mark it out in different colors.

Note: This will most likely be harder to do if you don't travel through cities.

States.pdf
Download

Step 2: 20 Questions

Most people are probably familiar with this game, but it's another game we like to play on the road.

There is even a wiki page for 20 questions . It goes into detail about the history of the game as well as a few variations.
Here are the instructions for the basic version from the wiki.

"Twenty Questions is a spoken parlor game (or car game) which encourages deductive reasoning and creativity. It originated in the United States and escalated in popularity during the late 1940s when it became the format for a successful weekly radio quiz program.

In the traditional game, one player is chosen to be the answerer. That person chooses a subject but does not reveal this to the others. All other players are questioners. They each take turns asking a question which can be answered with a simple "Yes" or "No". In variants of the game (see below), multiple state answers may be included such as the answer "Maybe". The answerer answers each question in turn. Sample questions could be: "Is it bigger than a breadbox?" or "Can I put it in my mouth?" Lying is not allowed, as it would ruin the game. If a questioner guesses the correct answer, that questioner wins and becomes the answerer for the next round. If 20 questions are asked without a correct guess, then the answerer has stumped the questioners and gets to be the answerer for another round."

There are electronic versions of this game now available. The "computer" is the questioner and you are the answerer. While the electronic version may be fun for a while, I think it's the family communication that really makes it fun.

Step 3: Famous Quotes

In this game, we simply take turns saying famous quotes and everyone else attempts to guess where the quote originally came from. They can be quotes from anything famous, but we usually use movies or TV shows that most people in the car are familiar with. Other examples would be famous presidential quotes, or recognizable quotes from professional athletes.
We do not usually keep score, but that would certainly be an option. I usually learn a few new quotes each time we play.

A few examples:
"Run Forrest, Run" - Forrest Gump
"I'm not a crook" - Richard Nixon
"Float like a butterfly, sting like a bee" - Muhammad Ali

Step 4: Catch Phrase

This is a commercial game that is intended for two teams to compete, but for road trips, it's just fun for everyone to guess. The game is based on 1 clue-giver attempting to get everyone else to say the given word or phrase, without actually saying the word in any of it's forms (e.g. past-tense). You also cannot say rhymes with, or starts with. You must use other words or phrases to give the clues.
We use the electronic version which has an automatic timer built in. We attempt to get through as many words as possible until the timer expires.
We usually give ourselves challenges to make it more fun, while still competing on the same team. This would work by stating that we must correctly guess 5 words in the time limit, 10 times in a row. If we guessed less than 5, we would start over. It's fun and helps pass the time.

Note: While this game includes the driver being a guesser, clearly the driver should not be the one holding the clues and should keep their eyes on the road.