The Koopa Hotels were seven different hotels scattered around the Mushroom Kingdom during Bowser's take-over events in Hotel Mario. Mario and Luigi had to go through each of the hotels to save Princess Peach and the kingdom. Each hotel was owned by a different Koopaling except for the last one, which Bowser owned. Strangely, Iggy Koopa did not own his own hotel and shared it with Bowser and appeared as a mini-boss in it.
These are all the hotels in the game: