- An empty room, especially one with a lockable door offers shelter for the player characters. That's where they can hide. After all, if the monsters know, that the room is empty, they might not bother going there. Except to hide away from other monsters, they want to evade... So, do not bash in the door. Let your thief unlock, and lock the door, instead.
- An empty room can be filled. Move some of the furniture in there, like that cozy armchair from the hallway. Put it in front of the fireplace, get the fire going, and prepare a cup of tea. Let your cleric create food, biscuits if possible. Enjoy tea-time.
- You know, when you finally conquered the dungeon, drove away goblins, and chased nasty spiders out, or to the lower levels - do some spring-cleaning and move in! What, you never thought about that one? With housing cost, and real-estate prices rising everywhere? Get a dungeon!* It's like a hobbit hole, especially if you like visitors. You can lock doors, and barricade entrances to the lower levels, or some portions of the dungeon, so you don't have to worry about the whole area. Nor, will you have to go around dusting all those shelves, remove spider webs and so on.
- Use empty rooms to build camp. If several rooms are available set up different camps. Leave supplies, and some tools there. Spare weapons, and healing potions, for example. Secure the rooms with traps. Of course, you can. Everybody does that in a dungeon.
- Should you happen to find 2.000 copper pieces use them to play games, seed a trail through the dungeon (luring monsters where you want them to go), throw them down into that fountain, and make wishes. What's the fuzz about 2.000 cp in an empty room? There are so many uses you can put both to. Don't complain, play!
- Build a nest for love birds. I mean, really, isn't it sort of romantic to slip away with your flame into the dark dungeon, and occupy that empty room? Share ideas on how to furniture it together. Find the best, and safest route in and out of the dungeon.
- Fill it with lots of colourful balloons, or squishy balls. Dive in. It's fun. Monsters are gonna love it, too. Just wait until the word has spread, and you've got the rest of the dungeon for yourselves.
*Actually, that makes for a nice encounter. NPC parties who occupy the dungeon to prepare to move in. And who will get all that treasure?