What games can I play with my non-gamer girlfriend?

So, my girlfriend isn't a gamer, but she's open-minded. I was wondering, what does Reddit think would be some good games to introduce her to it? I'm not going to try and make her become some hardcore gamer, but it would be fun to play some light games together.

Free is encouraged, multiplayer would be nice, PC is more convenient for me. I was thinking that Portal was a good idea.

Consoles : Wii, PS2.

She's played : Smash Bros Melee, Mario Kart, Dead or Alive Beach Volleyball Extreme (Yeah.)

inb4 : "HEY GET A LOAD OF THIS GUY HE HAS A GIRLFRIEND"