As far as good traveling games (easy to play for long periods of time, with good depth, yet not too bulky of games) go, I'd have to highly reccommend the following:
~ Mega Man Battle Network series. These are great games, great music, fun gameplay, lot of game time in em, and a lot to stuff to do in em even after the game is beat. 2 is personally my favorite, but from what I've seen most people like 3 and 5 the best.
~ Mega Man Zero series. Really fun sidescrollers, not overly long, but still have a good amount of game time and development in them. As always, fantastic music. Best to start from the beginning with #1, they're all really good.
~ Fire Emblem series. Awesome RPG/Strategy games. Similar to Final Fantasy Tactics (which is another good choice BTW), but with a lot more depth and character development options. These games have MASSIVE character development with around a hundred characters and multiple ways to upgrade them all. Lots of replay value. The Sacred Stones would have to be the best one IMO, best storyline, character development, length, and difficulty balance.
~ Wario Ware series. Wario Ware is a great mini-game game such as Mario Party, but meant for single player. I warn you, this game is HIGHLY addictive, and has a massive amount of gameplay in it. The first one for GBA is definitely the best.
~ Pokemon series. If I have to explain these games to you, you live under a rock. They are amazing, and you should know that by now.