If cleaning the bathroom is actually one of your chores already, then substitute some other necessary, but mildly tedious one. Anyway, just clean it and don’t tell her** ahead of time that you’re cleaning it. That includes cleaning the shower/bathtub and washing the mirror, by the way. All of this will take you half an hour, tops. Do it when she’s doing something else, then let her notice on her own.
Happy Valentine’s Day!