Emporio's Room

Hidden in every city throughout the game (barring the foggy town where you fight Justice) is the entrance to the Ghost House, or Emporio's room. Sometimes, when you face a wall that appears to have nothing there, you'll hear a knocking sound. Knock a couple more times to be let in. These cannot be accessed while playing as Josuke. There are a couple things of note in here:

If you try to touch the computer on your first playthrough, Emporio will ask you not to do so, but on a New Game +, you can use it to access the Stand Summoning Program. If you talk to Emporio after saving a code in the first slot, he'll give you his own, allowing you to battle him.

By inspecting the sink, you can receive an infinite amount of Bottled Water if you're patient enough. The door on the left leads to the Sanctuary, where you can speak to the ghosts of enemies you've defeated and fight a few secret bosses once your level is high enough. The door on the right won't lead anywhere for most of the game, but checking it after locating Dio's mansion and returning to Cairo will take you to the Rolling Stone Maze.

A list of entrances to this room in each city is below: