Back in ye olde 2006 (update) the Construction skill was released. Houses were historically a major social area, people would throw house parties to celebrate achievements or to have people run through their own dungeon. You can make a kitchen and cook in it, a dining room or sitting room for sitting in, hire a servant or butler to greet guests and serve tea, a menagerie to place the variety of pets in-game in, an aquarium to place eggs you find while fishing in and decorate, then you can dive in and look around your aquarium. You can also build a chapel to place an altar in, which allows you to restore your prayer points in your house but also provides the most popular form of prayer training in-game. There's a portal chamber that you can attune to various destinations, and and a study for creating teleport tablets and using a telescope to find fallen stars. There's also a costume room that allows you to store sets of gear and capes in. Some of these have become more and more niche over the years, but ultimately it's still got a fair amount of depth.