Jonathan Swift wrote a poem titled, "The Lady's Dressing Room". Wikipedia describes what the man finds when he sneaks into his lover's dressing room, "He finds sweaty smocks, dirt-filled combs, oily cloths, grimy towels," and so on. Worst he finds the chamber pot.
Consider this scenario. The party finds a painting of a beautiful woman in an elegant gown. As they advance through the mansion, they find the lady's dressing room. Instead of elegance, they find dirty clothes, grimy towels, used feminine products, and a smelly chamber pot.
Dressing rooms are not exclusive to women. Men can have dressing rooms. Consider the noble knight. Where does he put on his armor? Does he strap on the plate mail himself or does he have his squires help him? Depending on the personal hygiene of the knight, his dressing room could be as bad or worse.
Dressing rooms would have lots of shelves and places to hang clothing. Where does Superman hang his clean Superman suits? Would they be out in the open, or hidden behind some secret panel in his dressing room? A dressing room is an obvious place to have a secret panel to hide your alternate identity or sexual fetish costumes.
Consider this scenario. The party searches the dressing room and finds a secret passage into an adjoining secret dressing room. The room contains a set of skin-tight leather armor. However, the armor has opening in the crotch and provides no protection for the gentiles. Wearing the armor provides +3 to your sexual attractiveness.
What dressing room would be complete without at least one full-length mirror? In the movie Legend, Mia Sara dances with the mannequin in the black dress. Suddenly, the dress appears on Mia Sara's character. If you hold up a piece of clothing and stand in front of the mirror, the mirror could magically show you how you would look wearing the clothing. Alternatively, the mirror magically teleports and transforms your clothing.
Depending on how diabolic the Dungeon Master wants to be, the mirror could change the gender of the character depending on the type of clothing. For example, if an adventurer picks up a dress and the adventurer happens to reflect in the mirror while holding the dress then the mirror could change the gender of the adventurer to female and outfit her with the dress.
All dressing rooms need a place to store shoes and boots. What evil overlord is complete without a collection of highly polished boots? Imelda Marcos was infamous for her huge collection of shoes. No shoe collection is complete without at least one pair of magical elf-skin boots.
Mannequin are the perfect place to show off special outfits. For example, a female mannequin wearing a chainmail bikini, or a male mannequin wearing a chainmail jockstrap. How lifelike are the mannequin? Were they once alive and transformed into a mannequin? Do the mannequins come alive?
Dressing rooms often contain a variety of substances to improve the appearance of the owner. In the movie, The Abominable Dr. Phibes, Vincent Price puts on his wax face. Imagine the party finding a drawer full of various types of ears and noses.
Every dressing room should have at least one table for various creams and potions. For example, skin cream made from real skin. The table could have lots of pretty tins and boxes. Bell jars filled with a variety of preserved objects can add interest.
The key is to make each area interesting to the party. The area does not need to be harmful or helpful. The party needs to have a tale to tell when they get back home.