The bathroom is the last room anyone decorates for Halloween, which is exactly why doing it lands. Guests are not expecting it, the room is small enough to change completely in an afternoon, and nothing in it has to survive a party.
The version worth doing is not cobwebs and a plastic skeleton. It is the room going dark, warm and slightly apothecary for a month.

Change the towels first
Textiles do more than props here. Pull the white towels and put up charcoal or oxblood ones, and the room reads as deliberate rather than dressed. They go back in the linen cupboard in November and get used all winter, which is the argument for spending anything at all. Our bath towel guide covers weights that actually last.
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Take the bulb down, not the light
Overhead bathroom light is the enemy of every mood you are trying to build. Swap to 2700K warm bulbs and the same fixture stops feeling like a dentist. Add one rechargeable picture light above the mirror and you can turn the main light off entirely, which is when the room finally looks like the photo. The reasoning is the same as in our layered lighting formula.
An apothecary shelf, not a haunted one
The mirror is worth a moment too. If yours is a builder-grade frameless sheet, a length of dark trim or a leaning smaller mirror in front of it fixes the room faster than anything you can put on the counter.
Amber and smoked glass does the spooky work without a single plastic object. A few amber bottles with dried stems, a black taper in a brass holder, one dark stone tray to hold them together. Group in odd numbers and leave real space between the groups.

Dried, not fresh
Fresh flowers in a guest bathroom are a three-day commitment. Dried stems in a dark vessel last the whole month and read as moody rather than sad, and a length of eucalyptus over the mirror frame gives the room the one soft line it needs.
A dark tray under the group is what turns three objects into an arrangement. Stone, slate or a dark wood board, small enough that the counter still reads as mostly empty.
Scent is half of it
A bathroom is small enough that scent carries instantly. Something smoky, woody or ambered rather than pumpkin. A single lit candle beats any plug-in, and our candle and diffuser guide has the ones that fill a room without a headache.

Hide the ordinary things
The fastest upgrade is removal. Toothpaste, bottles and the hairdryer go into a closed basket for the month. Empty counter, two styled objects, warm light. Our bathroom storage guide has the pieces that keep it that way after October.
Availability changes often, so check before you buy. Want the whole house planned rather than one room at a time? Start with our Room Recipes, and our Halloween decor guide covers the rest of the rooms.