Taking the girlfriend to her first game tomorrow.
I'm not a serial attendant by any means but I've been to a fair share of games and have experienced Wrigleyville many times.
That being said I feel like you start to take things for granted and forget all the little things you made sure you experienced your first couple of times.
Suggestions on things I absolutely need to show her?
The first times I went I enjoyed walking around the outside and looking at the statues (Ernie Banks, Harry Caray, Ron Santo).
A picture in front of the red marquee is a must, obviously.
Assuming you're both in your 20s, show her around the Wrigleyville neighborhood. You don't realize what a one-of-a-kind atmosphere it is until you visit other stadiums. Maybe try to get into the game a little early to walk around the stadium, take a picture of the ivy, etc.