Not really knowing what you might have on hand already, or if you want to buy items to prepare would help very much. But to start with, you have to have some type of diaper. Have it, and the things associated with diapering ready to go. I would say, baby powder, baby oil, wipes, a pacifier, baby bottle, are just some basics you can easily get anywhere near your home.
If you want to keep it intimate, and just for the two of you at home. Tell her, she seems to need some regression treatment, and will need to wear and use her diaper. You will be in charge, and she will do as she is told. Now, just do whatever might come natural. Play baby games, play dolls, watch children's TV, do a bottle feeding, play dress up. Once you have her in a diaper, and relaxed with the situation, ask her what she'd like to do. Take any ques from her, she might let you know just what she wants out of the time you are providing.
I think it's great you want to treat your girlfriend to such a surprise. I hope this small bit helps you both out. I hope you let us know, and share how things go?
If you have any other questions, don't hesitate to ask. And maybe you can tell what you do, and don't have to start with.
Good luck, good luck to the baby, and enjoy!!!