We have the game Rubberneckers, which is basically items to find while on the road (a car with more than 3 bumper stickers, a US President's name-not Ford, a toll free phone number, a windmill). You could make up your own list of things and assign points if you wish. Or find a travel bingo- maybe there is a free printable one online.
We have also looked for a semi in every color of the rainbow (purple and yellow can take a while). Once my DD tallied semi colors all summer. It was interesting to see which colors are the most common.
Most license plates around here have a set of 3 letters. We try to come up with phrases that start with each of those letters. For example, MOZ can be My Own Zebra. The 5 year old may not be able to come up with her own, but if the adults can make them silly, she might still enjoy the game.
We've done guessing games, like thinking of a Disney princess or character and either give a hint to start or just let the others ask yes/no questions.
Finally, a good travel toy is Crayola Model Magic. It's similar to playdoh but completely clean and doesn't fall apart or crumble. You can mold it into bunches of things, then let it harden, or put it away for later. Blending two colors into one new color completely takes a good amount of time, or you can blend them into pretty swirls of color.