My wife got a DS for Christmas, and is rapidly growing skeptical that she will use it. I would describe her as a semi-casual gamer - she thinks that things like Nintendogs are stupid, and is lukewarm at best on Brain Age, but on the Wii she *loved* Elebits, and has had no shortage of enjoyment with Wii Sports and Wario Ware. She also expressed interest in Super Paper Mario, but hasn't gotten around to playing it, so I can't say for certain that she liked it.
She is as uninterested in something big like Zelda or Final Fantasy as one can reasonably be.
She has Cooking Mama, which she seems somewhat pleased with, and I know about Wario Ware Touched. What else would you recommend for her?