Bathroom slippers
In Japan, there are two “bathrooms:” the first contains a toilet and a sink, and the second has an actual bathtub. Regardless of which one you use, you should always wear special bathroom slippers. They’re usually inside of the bathroom or just outside of the door’s threshold.
Since you’re walking around in socks, you don’t want to get them wet, and wearing these slippers will prevent that from happening. Also, make sure that you take them off when exiting the bathroom. People might snicker at you if you wear them outside. Also, they’re pretty dirty, seeing as how people share them when using the bathroom.