I would recommend Alfred Posamentier's Advanced Euclidean Geometry (Key College Press, 2002). It covers much of the same topics as Geometry Revisited by Coxeter/Greitzer and Episodes... by Honsberger, and it also presents accompanying technology (namely, Sketchpad applications) that allow the students to play around with the results. That is, it gives the students more opportunity to learn how to think geometrically.
I love all of the texts mentioned (Altshiller-Court, Coxeter, Coxeter/Greitzer, Honsberger,...), but their approach to the material is very different from what undergraduates would be used to. And very different than what they will be teaching.
You might find yourself spending a lot of time helping them process the text material into concepts they would find more natural, particularly, if any one of these were the primary text of the course. This may be more work than you would have originally desired: not only teaching the mathematical content but also how to translate mathematical texts.
Then again... this is a good thing for a high school teacher to know how to do...
Either way, I strongly recommend that you look at the Common Core Standards for geometry (at http://www.corestandards.org/the-standards/) and familiarize yourself with what content these future teachers will expect to be teaching. From there, you can get a good idea of what type of thinking and content knowledge you feel that someone would need to teach this material excellently.
