Hi!
The game my family has always played on long trips (or short ones too!) is the A-B-C game.
It's where you look for signs that begin with a certain letter. Like for the first letter, A, you'd look for a sign that starts with an A. And then for the second letter, B, you look for a sign that starts with a B. And the words can be on anything, too. Street signs, business signs, even bumper stickers! It can get very competitive, but it's fun for all ages!
And I have a sneaking suspicion it's good for learning the alphabet too. But, like I said, it's fun for more than just the kids.
-Alaskaluver