One of the lessons I learned early on in creating Dwimmermount was that most levels, if not every level, ought to have some locale or feature that encourages return visits to that level. So, there's the "Cleric Tree," a weird tree-like plant that bears fruit that act as potions of healing and the Moon Pool, whose waters have various potentially beneficial effects. These locales and many others scattered throughout the dungeon encourage repeat visits and provide the referee with the chance to restock rooms with monsters and treasure. They are, in my opinion, a vital part of keeping a dungeon "fresh" and making it feel "alive."
For today's Open Friday question, I'd like to ask people to share some of the locales in their dungeons that players return to again and again, even as they explore deeper into them.