One more suggestion that no-one's mentioned yet: I have always found passengers quite unnerved by the sensation you get when you retract the flaps. It's something that I've not really noticed since my very first lesson but people definitely feel something.
I always say something like "you might notice a strange sensation now, while I retract the flaps". That's about the best way I can describe it, but it definitely helps.
In general, I'd advise you make some comment about anything that you change, before you change it, especially power.
My vote is for the front seat: that way she'll see what you're doing and she can also see your face. Being able to look calm and smile a lot relaxes people enormously.
If you do put her in the back, don't let your front seat passenger 'have a go'. If anything will scare her, that will.
Mark