Note that Trauma Center is more of a game than Cooking Mama. The latter is fun, and cute, but less complicated and such.
Also, I hear that Chessmaster XI: The Art of Learning isn't too bad. I've been playing it on PSP, and while the subtitle is basically, uh, wrong, there really aren't good portable chess programs out there other than that one. Of course, this depends entirely upon whether they like chess.
I believe there are a couple of crossword games which might appeal to them if they're into that. I forget the names, but I'm sure they'd be easy to find.
That's about all I can say. All the games I have for it are games. I'm not all that interested in the non-game stuff. Though I guess you might also consider Professor Layton and the Curious Village. I know, it sounds like a kid's game, but it really isn't. It's got a lot of very interesting puzzles and, depending on your folks, might be something that would appeal to them. Aesthetic and plot stuff regardless (which is very quaint and unique), the game is mentally stimulating and big enough to keep you going for a long while. It's one of my favorite DS games.