Here are some games friendly for hardcore and casual players alike:
Plants vs. Zombies (a good towerdefense-like game)
Super Scribblenauts (good game if she's into solving puzzles on pure ingenuity)
Phoenix Wright: Ace Attorney (a must, ...seriously, a must)
Kirby: Squeak Squad (if she likes platformers. I chose this instead of Super Star Ultra as this one is good for those new to the Kirby franchise)
Ghost Trick (a fun game from the creators of Phoenix Wright. You recently died and you must use your ghost powers to investigate how you died. Detective-like.)
Animal Crossing: Wild World (a lot of people like this game so why not?)
Rhythm Heaven (a rhythm game with cute japanese-doodle characters, nice songs and challenging gameplay)
Elite Beat Agents (a really fun rhythm game with popular American songs)
WarioWare: Touched (game which contains many small minigames which are quite funny and quirky, my family loves thisfranchise)
Nintendogs (any version of her own dog breed preference, cute game in which you take care of a puppy)
Super Princess Peach (an exciting platformer in which you play as Princess Peach from the Mario franchise. Plays like Super Mario Bros but with Peach and her special powers)