Heh, good luck finding an "Adventure" game. To my knowledge, the only true adventure games are Shenmue, Beyond Good & Evil, Megaman Legends, and Zelda. And there's only one Zelda game on the DS right now.
But anyways, there's lots I can recommend to you;
Echoes of Time, I would NOT recommend, unless you've got someone to play with during your trip.
Talking SRPGs, if anything, go for Knights in the Nightmare. It's a little difficult to get into, having a relatively steep learning and difficulty curve, but provided you look at all of the tutorials in the game and actually pay attention to them, it shouldn't be that hard. Get this, and you've got a very solid SRPG with a very good, albeit somewhat confusing story. Just not as confusing as Killer 7.
There's also Super Robot Taisen OG Saga: Endless Frontier. Unlike the name suggests, there aren't really any Super Robots in it, but the battles are simple, and easy to get into, and the overall game has the feel of a mixture of a SNES RPG, and something made with RPG Maker. Plus, it's got Reiji and Xiaomu from Namco x Capcom, and Kos-Mos and Shion!
If you haven't played any of them before, I'd also recommend Megaman Star Force. It's not quite Battle Network, but they're still very good games, having characters that don't quite stand up to the likes of Netto, Raika, or Enzan, it's still got a solid cast of characters. I dunno if you played the first two, but the third one actually just came out today...
Shin Megami Tensei: Devil Survivor is also right up there with my list of DS games to recommend; it's got everything that ever even made MegaTen liked by its fans, and some things new to the series, having a fairly simple battle system, and a solid story with memorable characters, and a good 6 endings. And unlike Persona 3 & 4, your main character has expressions!
Also, they're not very easy to find, but if you do, they're relatively cheap; try to pick up one or two of the three DS Castlevanias. They're all very easy, and can be finished in a matter of days, but they should be able to satisfy your needs for your trip.
And then they're not really RPGs or SRPGs, but more Action type, I'd also recommend Ninja Gaiden Dragon Sword, Megaman ZX, or Megaman ZX Advent. Megaman ZX and ZXA are both pretty much what Megaman has been for the last 21 years, but with a mix of Metroid's free-roaming worlds. The worlds just aren't as interesting, but the stories for both are pretty decent. The only thing that might be a turn-off is that the difficulty can occasionally make Megaman Zero look easy.
Ninja Gaiden DS is... Fairly interesting as well; you hold the DS sideways, and control all of Ryu's movements from the "Right" side, the touch screen, and you get a map on the "Left" top screen. The game isn't nearly as difficult as its prequel and sequel, but it has its moments.
There's also Chrono Trigger DS if you haven't played it. While I prefer the other versions, availability and playability is the issue in CT's case, as the PSX and SNES versions are... kinda unplayable on the road, and are difficult to find for reasonable prices, along with whether or not you have interest in it. I'll refrain from saying anything on it, since it's difficult for me to say anything without going out on a tirade on just how amazing it is. I'll say this much; the excuse for most of the events in the game are described as the predestination paradox, although not outright. You need to have some understanding of the concept before you go out and yell at the game saying "Snake, you can't do that! You'll create a time paradox!"
And no, I couldn't pass up a chance to quote Snake Eraser.
Hope I was at least able to help here though, and I hope you have a nice and safe trip!
