You didn't mention any ages, so I'm just going to list a few of our favourites for a wide variety of ages. With more specifics, I could possibly provide other idea options.
We really enjoy
Animals Tic Tac Twice. We have carried it with us for years when we travel with kids. It takes two to play. Tic Tac Twice is similar to Tic Tac Toe, but has a bit more strategy (though not too hard for kids), where you have to get four of your colour in a row. The game is in a CD case, so easy to travel with no matter what mode you are using.
A friend of mine always packed a baggie with cotton swabs that had been pre-moistened with water, and then let her kids do those "paint with water" books in her car because they were non-messy that way.
We have an Areaware Cubebot Micro (Black), which is a manipulative guy that can be puzzled into a cube, which gets a fair amount of play, plus it is tiny. My son also enjoys the Transformer-type robots, which are puzzle-like in their own way. The more moves to go from vehicle to robot and back, the greater the difficulty.
For kids who are able to read (as well as adults, not counting the driver!!!) Rubberneckers has been good.
Older kids and teens get a few Hanayama puzzles tossed in. There are different difficulty levels to choose from. They are slightly pricey at about $10 each, but are very good quality.
The Paint By Sticker for Kids series of books are good. (They make harder ones for older kids and adults).