Whenever anyone in the movies gets super powers, the first thing most of them do is prank somebody. You haven't been here long enough to really hate anyone on a personal level beyond a few standout cases. Just confusing the shit out of her should do nicely.
"Everyone except Ms. Janett doesn't notice the changes I make to her, until I want them to." There, generic enough that it shouldn't backfire. It still probably would, you've seen the movies, but that little safely gate should prolong your fun a little bit.
You participate in the workout for the fuck of it, running laps as she adjusts the way she's holding herself. You haven't really done anything yet, and she's already so touchy. Imagine how she'll get when the big changes happen. Hell, forget imagining it.
"Ms. Janet slooowly becomes thirty years old. Physically, not mentally. Well like- She doesn't loose muscle- fuck the last command. Ms. Janett slowly becomes like thirty year old. Over the course of five minutes." That was probably the most confusing wish so far. Maybe you might wanna plan them out better, like writing it on paper before you put your foot in your mouth and accidentally blow something up.
Didn't want it to slow, or you'd die of boredom before anything happened. Still, you got a good view as her fidgeting increased. She started rolling her shoulders, tapping her legs more. You could see the tan leathery skin slowly whiten, stopping at a still tan shade and looking marginally better. Her hair, on the other hand, you could barely tell the difference. If you weren't looking at it, the change from white to platinum blonde was invisible.
With her stretching, you could see her breasts rise slightly and go back down less. With perky tits under her long sleeved running jacket, she still didn't look to much different another then confused. She was still stretching, perhaps noticing how easy it was to move. B-cups bounced as she smiled, albeit a little confused. It was the happiest you'd seen her in, ever. To bad your tomfuckery wasn't done yet.
Next change, you decide to...