Opportunities abound to stay for free at someone's home or apartment while they, themselves, are on vacation (often pet sitting or plant-watering are the only requirements)
Sleep for free on vacation by watching someone's house (and, often, watering their plants and feeding their cat)
In that paragon of travel documentaries, National Lampoon's European Vacation, there is a scene in which Ellen Griswold (Beverly D'Angelo) forgets to lock the door on the shared bathroom in their London hotel. A man comes in to brush his teeth and is pleasantly surprised when Ellen—washcloth over face and believing the man to be her hubby, Clark—calls to him from the bathtub to join her. Shock and hilarity ensue. The accidental intruder was played by none of ther than Robbie Coltrane, 16 years before he embodied Hagrid in the Harry Potter films.
