It sounds like she has no idea what she likes. As a woman, I can tell you it's very helpful to have to good handle on how you feel and what you want before you try to do it with other people.
A couple pretty safe ideas that can make a good girl feel naughty enough to at least give it a thought:
1. If she doesn't have a vibrator, she should have one. If you get it, you need to not make a big deal about it. Maybe just leave it somewhere for her with a note about wanting her to be happy and feel good. It should be smallish, light colored, and non-threatening if she's prudish about using one. You would also need to give her lots of time at the house alone
2. Passion Parties (aka tupperware parties for sex toys) are a not-very-naughty thing that makes prudish girls feel very naughty. If she doesn't know anyone that does them, it may be hard to hook this up. But they're a very good idea.
3. 50 Shades of Grey is not that kinky. For vanilla girls it's "just kinky enough." If she likes romance movies or twilight type stuff, this could be an easy present. Because EVERYONE is reading it, you could assume from that that she might like to. Like I said, it's not all that kinky, but there are a lot of sex scenes that might make a good girl think of things she wouldn't otherwise think about.
4. Maybe just say to her "I don't think I'm giving you what you want sexually, but I don't know what that is. So I'll tell you what, you tell me what you wish I would do for you in bed, and I'll try it for you, next week/month/whatever." That sounds nice and might open her up or at least get her thinking about it. Even if she doesn't have an immediate answer, it might give her something to think about.
Basically, I would just give her the time, presents, and non-pressuring encouragement to figure out BY HERSELF some things she might like, so she could then bring it to you. She needs to have her curiosity inspired and be free to explore it before acting on it. I've never been vanilla, but I've always been female, and I know if someone wants to convince me of something, they need to give me the time and opportunity to figure out how I feel about it first. Then the curiosity is your own and not something you're just doing for someone else