Any Mario Kart game is recommended. For road trips, where I'm not driving, since driving and playing games at the same time would be a terrible idea, or plane trips, or any other kind of travel, I like to play something simple enough to get into, without having to commit to it. While I'm fond of jrpgs for example, they're not normally my first choice while traveling.
Try something that doesn't require long play sessions to get the most out of it. You may not want anything with super long cutscenes, and I would avoid games that only allow you to save at certain points, because you could have to stop at any point, due to arriving at your destination, or at one of various pit stops along the way, so you should be able to at least be able to create a temp save, and quit at any time. Modern systems have a sleep mode option, but it's still not a great idea to get into something like a hardcore dungeon crawler with sparse save points. Mario Kart games are great for traveling, IMHO. Maybe try some kind of quirky indie game that's easy to pick up and put down. Depending on how long your trip will be, something story based isn't a terrible idea. Just be sure to have some idea when you'll have to stop, so you can plan your gaming sessions accordingly.
I have made exceptions to my rules in the past, mind you. If I'm ever currently playing a handheld title, I often want to keep playing it, no matter what it is. Since I'm more of a console gamer, more often than not, I'm actually locked into something on a console, which I'll be away from, so that's usually not an option, and therefore, isn't a problem. Sometimes though, I really want to play a new game I acquired, so I start it up to see what it's like. As someone with a huge backlog, I can relapse in and out of gamer's ADD, and sometimes rely on my mood to pick what I want to play, which could clash with my usual rules. But, if I were to pick something to play, based on my travel criteria, I definitely lean towards something simple, with frequent save points, if not the ability to save anywhere.