hmmm...i don't know of any other RPGs, because i haven't tried any for the GBA. but i do know zelda is a great game. have you played it on the snes before? either way, the game is awesome, so if you want to, you could always play it again.
i'm trying to stock up on games too, so i don't have much. so far, i have:
sword of mana - it's okay. but then again, nothing is as good as secret of mana. i haven't played the game much...so i'm not sure how much you would enjoy it. if you check on gamespot.com, they have reviews. the user reviews are usually pretty good at finding the better games for you.
yoshi's island - heh. yah...i know it's not a RPG. but it's a game i have, so i'm listing it. it's fun, except it gets annoying after a while (esp. trying to unlock everything.) probably not a car game.
wario warez - ah...do you have this one? this one is really fun.
i like this one because it would be good for a car trip.
metroid...something (the first one for GBA) - bleh...i guess my expectations are too high. i didn't like this one that much.
i really don't know any RPGs for GBA. maybe you should check out ff tactics advance? i don't know. i have the one for PSone, and it's pretty neat (just started playing it). but it's more of a stragety-type game, so...
or you could always check out some of the older games (GBC or the original GB)
sorry that i didn't give you any ideas. i just wanted to reply.
