Slippers unless I'm staying in a hotel that will provide them. A couple times I've similarly taken a light bathrobe. I always bring some soap, shampoo, and shower gel or bubble bath that I like.

Some luxury hotels will arrange (for a price) to deliver a Christmas tree to one's room/suite.

Convention/conference hotels can arrange to deliver a white board or flip chart and markers for meetings, also for a fee.

I've seen kids' teams decorate their doors with colored paper in school colors and pictures.

I used to be the typical traveler who gleefully brought home the mini-bottles of hotel toiletries, thinking it a win because then I didn't have to buy any. Ten years ago I turned that around 180° and starting bring my own, quality toiletries from home. A small set of reusable bottles I bought enables me to fit shampoo, scrub, hair gel, toothpaste, moisturizer, and mouthwash in a "3-1-1" bag with plenty of room to spare. Having high quality, familiar products for my grooming routines creates a level of consistency and comfort-- i.e., a sense of home-- regardless of where I am in the world.

As for slippers and a robe... I don't wear those at home, but the broader point is valid: Pack comfortable clothes for your downtime. For me that's generally shorts, a light shirt, and light wool socks. I change into them and hang up/fold my business clothes when I reach the hotel and have no more client-facing responsibilities for the day.

Slippers, tea kettle and teas/coffee, thermos mug, Comfy PJs, clothes for downtime as mentioned above. Depending on location, a thin travel robe or a fine cotton shawl/pareo. If I remember, a folding vase to put flowers in.

If I'm on an extended trip, I might add travel spice container and small cooking supplies and another travel appliance. I usually enjoy eating out but if the weather is bad, I'm too tired to go out or I need something healthier I stay in my room and cook.

Amen to this; great idea.

I always like the scents of home; candles and/or diffusers.

An inexpensive plastic shower head (about $5 at Home Depot), and you'll need a pair of pliers to remove the clogged one in the room. I've been doing this for more than 30 years now, ever since I stayed in a room where the shower head was so badly clogged I could barely get wet. Could have been water conservation gone mad since it was in California.

Hints:

1. Don't forget to take your shower head with you and put the original back. They won't suspect a thing.
2. Measure your pliers. If they're 6" or less, you can keep them with your carry-on baggage.

Flowers. Often placed in an empty beer glass. Mostly bought from discount / day-old bucket in random supermarket as I will not be there very long.
Sometimes a cheap mug from a dollar store or similar to call my own.

I bring slippers,robe, if I am able I’ll buy bathroom spray and bathroom cleaning spray at the location or if I am driving or taking ground transportation I will bring it. Baggage cost too much so I keep it minimal.

Nothing to personalize but I do bring a night light for the bathroom, it make finding it easier in the middle of the night and it saves from turning on the super bright lights in the bathroom and shocking the eyes, I also started bringing a roll of electrical tape to cover all the LED's, the microwave, TV, alarm clock, etc..

I might go to a scented candle. Most of my longer stay preferences tend to be settling on my clothes, toiletries, and room arrangement a little more to my liking. I might rarely bring a pillow from home too.

I bring pillows - use a vacuum bag for them (I like the cheap cheesy Wal-mart ones so 2 = 1 normal pillow). This trip I needed a little cheering up, so picked up a pack of those battery tea lights and found these white paper bags so I have a few lanterns around the room.