Well there's plenty of games for the 3ds that are great.
For starters Kid Icarus Uprising is an awesome game. The controls are a bit weird but the story and characters are phenomial. If you can get past the controls the combat is pretty good too. The one thing I will say though is that it is probably not great to play in the car though as the way the camera moves would definitely cause me to get motion sick while playing. I mean if that's not an issue you face however than it's certainly a good one. I would recommend you play with headphones or something then though as the voices and constant dialogue are a really great aspect of the game and is something that you will want to listen to.
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@Marc said FE is pretty good. Not my favorite RPGS (I like pokemon and Xenoblade 2 better) however I find both FEA and FE Fates to be fun, challenging, and have compelling stories. The waifu thing is a little weird especially in Fates but you don't need to have any character marry each other so yeah.
What else....
OOT3d (Ocarina of Time 3d). I highly recommend this game. If you don't know it's the prequel to majora's mask and to me is one of the best games I have ever played. The story and the storytelling are absolutely phenomial, great and likeable characters, fun diverse and challenging dungeons (with the exception of the water temple). Also one thing I love is that they managed to make a zelda game intended for 10 year olds depressing and melancholy. In short if you have not played OOT I would like into obtaining this game.
Also Mario Kart 7 is pretty fun. While the character selection is pretty mediocre at best the race tracks are pretty good. There's not much else to say here honestly.
Smash 3ds is an option... but I mean if you have smash ultimate (or even smash Wii U) there's no real reason to get it. I stopped playing smash 3ds as soon as smash Wii U came out.
Also Mario & Luigi Bowser's inside story + Bowser Minions. I have not played this version but I have played the original and it's really really good. The bosses and enemies are challenging and pretty cool honestly. Fawful is a great and hilarious villain. Luigi provides great comic relief in that Mario pretty much has to force him to fight every single boss (which is quite hilarious actually). Also you can play as bowser!
On that same note Superstar Saga is apparently good although I haven't played it.
Metroid Samus Returns. It's hard to really explain why this game is good, but I personally liked it a lot. The combat is great, the level of progression is so satisfying as you slowly obtain stronger and better weapons. Even at the end with all these upgrades the game actually still manages to be hard. There's an ability that's let you view parts of the map and what blocks are breakable in the area you are in in case you get stuck. There's just a lot to like here.
But yeah I mean the 3ds has a great library of games despite not being that relevant any more so there's plenty of stuff you can try and play on that road trip!
Also don't forget that the 3ds can also play DS games, so that's a whole other can of worms to explore.
Hope this helps! 
