One of the biggest downsides to road trips is that they almost always consist of long stretches of road with little to do. While counting each cactus you see can help pass the time a little, there are tons of other ways to pass the time and have some fun on the road!
Deck of Cards
A deck of cards is by far the best thing to bring with you on your travels. They allow for multitudes of different gaming possibilities. While it can be difficult to play on the road, you'll still find many moments throughout your trip when a game of Crazy Eights or Go Fish can help pass a boring time. If you're bored with the usual fare, I'd look in to getting a computer game version of various card games and play with some computer opponents. I have the Hoyle Card Games program and it's been great in teaching me how to play some more complex games that my friends and I don't know how to play.
Portable Chess Set
I have a magnetic chess set that's about the size of a young adult novel. Not only is it wonderfully portable but it gives me tons of chances to challenge people to a game of chess. If the driver has a good memory and you both know chess terms, you could try playing while on the road. Just don't look at the board too long when you're driving! :-P Bored with chess or just don't get the game? Then try out some chess variants sometime.
When I was a kid my parents had a travel bingo set. It was SO much fun! I'd have my face pressed to the window searching for any object on the road I could add. There are also TONS of other travel games out there to play in the car. And I know what you're thinking. Just for kids, right? Wrong! There are tons of games out there that us big kids can have a good time with, too.
Playing Mad Libs is a(n) (adjective) way to pass the (noun)! It's (adjective) to play and it always makes everyone in the car (verb). You can make it sound (adjective) or even (adjective) if there's just adults in the car. All in all I (verb) playing Mad Libs!
I sometimes have a small pack of Trivial Pursuit cards. If I'm with people that know their stuff I'll read off a few questions, but most of the time I couldn't answer any of those questions even if I had the internet at my fingertips. Instead we play an Apples to Apples variant where we deal out the cards and each person calls out an answer with the answer cards they have on hand. A lot of fun!
1000 Blank White Cards
I LOVE this game! Basically make a set of blank cards and then make cards and play them. That's it. It's basically Fluxx but with an even more liberal mechanic.