I don't suggest teaching her words like "car" and "book" -- those are boring.
I also wouldn't beat the topic to death by turning a thread of conversation into some sort of sign language curriculum, so I'd do something brief with it and then push the interaction forward.
One idea that comes to mind is asking her name and then getting her to copy your lead as you spell it, "correcting" her hand signs so incorporate some light-hearted kino.
You can also start off by saying that you and your friends use sign language to secretly communicate with eachother and that there's no way you're letting into that "club". what has she done to deserve it? or you can explain that if you teach her too much, then you wont be able to hand signal to your friends if she's boring and you need a bailout.
