Keep it low level, I'd say 2nd to 3rd. Characters are tough enough to survive an unlucky hit and have a few tricks up their sleeves but not high enough level that they're complex and hard for a new player to understand. Also maybe let her play two of even three characters.
For the rest of it: ask your girlfriend what sort of game she wants to play. Give her options - combat, exploration, meeting NPCs, mystery, etc. The listen to what she says and give it to her.*
That's what I did when I introduced my girlfriend to pathfinder. She wanted characters who were good at social interaction. So we wound up with a bard, cleric of Ollidamarra and barbarian for muscle. She is playing all three of them. I take over one of them and provide a voice and some feedback when she needs to bounce ideas or just talk among the characters. They aren't the toughest group out there but are very good at talking to NPCs, winning friends and getting info. Even the half-orc barbarian has 12 Charisma.
For the actual adventure grab something low level from a Dungeon magazine or the start of an adventure path. Make modifications to it so it is more to your girlfriend's like.
And trust me, this works. At the start I had to wheedle my girlfriend into even trying "Dungeons and Dragons." She is now addicted to Pathfinder.
*This also works for birthday/anniversary/Xmas presents.
Cheers and good luck. If it works out, like it did for us, you too may find yourself gaming 2-3 nights a week with your best friend. (I know, so sweet it's sickening.)
Cheers,
Glen.