Well, I finished up SOTC, and that was the last ps2 game i wanted to play really. Unless the next persona game ends up being on ps2, bah. So I gave mine to my girlfriend. The first game she is playing is persona 4. I though a turn based rpg would be the best choice since it has little twitch requirements. But most of the games I have require a lot of twitch imput. Some people out there must have an idea of what she might enjoy. Something simple, but not childish if that even exists. Essentially she just wants for a DDR pad and singstar, which may be the best bet for her.

My first thought is The Sims, and with all the sequels and spinoffs for consoles, it wouldn't be hard to find one she likes. I don't know about RPGs, especially Persona, to start with. P4 takes a while to pick up and actually get into any real gameplay, but if she likes the story, she's good. 
 
Katamari Damacy and other platformers (Ratchet, Jak, Sly Cooper) come to mind, too. It comes down to how much gaming experience she has, and what types of games she likes.
 
Edit: Beat me to Katamari. Now I have the theme song stuck in my head.

" Now I have the theme song stuck in my head. "

okami maybe

She has katamari damacy, and we love katamari since she has my game collection. We played a bit of the new katamari for ps3 at a gamestop but she didn't seem to like it. I sold off my ratchet and jak games but maybe, but the combat might be a bit much. Okami is a good suggestion, I remember that not being extremely demanding. 
 
the problem with saying it depends on what she likes is that she hasn't played many games so i have no idea what types she likes. PS2 games are so cheap now that I can risk getting her a few. I played the first hour or so of P4 with her and she seems to like the story, I thought she would as she is a sucker for any J, K or filipino drama. But I think the whole persona collection aspect and whole grinding/dungeon aspect might get a bit overwhelming for her. There are so many PS2 games out there, I'm sure there are tons she'd enjoy.
 
Also, she hated Flower. Thought I should through that out there since people seem to think that is something a gf should like.

lego games would be an option if she like the stuff theyre based off of. Maybe batman, and definitely harry potter when that comes out.