One of the most common questions from people who haven't tried hypnosis in BDSM is: But she'll just obey me anyway, so what's the point of adding hypnotic controls?
Well ... here are three answers to that, scene enhancements that are hard to achieve otherwise.
1. Turn up the sensitivity. Tell her, "You'll feel everything twice as intensely as usual." The most ordinary contact with the most mundane parts of her body will trigger very satisfying gasps and jumps. After she's gotten used to that, say, "This is a melting ice cube" and run your fingertip lightly over her skin in a realistic melting pattern.
2. Turn down the sensitivity. Some subs would love to be marked up, but can't tolerate the pain involved. So simply instruct her, "You can't feel this flogger at all," before giving her a good workout.
3. Does she have fantasies of being kidnapped or abused by strangers? No problem, turn yourself into a stranger. Tell her something like, "As long as I'm wearing this hat, you won't recognize me at all." Then set up the scene, pop your hat on, and prepare to cover her mouth to block the screams.