Our house came with one of those when we moved in, pretty obviously built to be a fallout shelter (the concrete foundation on two sides, concrete blocks for the other two.) One light bulb, poor ventilation. Eventually we tore out the two inside walls and finished the whole area. There’s really only three things to do with a space like that.
Storage
Dungeon
Photography dark room
And the bonus is that unlike a hot tub, a cheese room won’t make the house smell like chlorine.
That’s the ticket. Install a pit trap, put a few devil’s-head sculptures with big O-mouths on the wall, some green slime and rot grubs, hire a few orcs and you’re set. Charge for stabling adventurers’ mules, and set up a first aid concession (cash only).
Infrared dry sauna’s are the way to go, not steam. You can buy a self-contained unit that you can take with you, or sell later at some point. There are plenty of health benefits to dry-saunas, plus not humidity problems.
Reading nook/library.
When folks visit, have something obvious they can pick up, a statue or something, while they check out the room that will start the obligatory rick-rolling. 
FFS I never noticed that. :smack:
As fun as a secret room sounds, unless you plan to spend a lot of time in there, it doesn’t make sense to pour a lot of money into the space.
I love the idea of a hidden wine cellar, if you happen to be oenophiles. A couple of store-bought racks and a cute table and chair set, and you’ve got yourself a neat little tasting room.
Sounds like the perfect place for beer to ferment.
Add a nice chess set and a comfy hide-a-bed and it sounds like paradise to me.
The ‘dog house’. My husband has a dog house. When he knows i am unhappy with him he retreats to his shop, out back. All his cool hobby things are there. He even has a sign shaped like a bone. It says ‘Pops doghouse’
This is the best idea so far.
Soundproof Scream Room, where you and loved ones can go to yell away all of your stress.
Love the idea of a wine cellar/tasting room. Have interesting and comfortable chairs plus a table made from a wine barrel.
I’ve been taking an art history class for a few years and right now we’re learning about some of the leading families in Renaissance-era Italy and the art and architecture they commissioned. The fashionable thing to do at the time was to have a studiolo, a small and gorgeously decorated room devoted to reading, reflecting, and writing. A marvelous studiolo hidden behind a secret door masquerading as a bookshelf sounds like enormous fun to me.
Speakeasy.
A dungeon. But not if it has typically low basement ceilings. That’s why we built our in the attic.
Yeah, I know. I was going for the Clue reference. This guy gets it…
Did anyone mention a secret lab yet? Meth, or Frankenstein types…whichever.
Make a “conspiracy” room – get some photo frames (that come with pictures), random newspaper clippings, and yarn.
Or a superhero/villain lair --just add CCTV monitor(s) and costumes
Brian
A fun one would be to get some special effects people in to build a horror / ghost sighting setup that’s hidden behind a bookcase on hinges. Whenever you have a guest over, open it slightly so it’s ajar, and if they go to open it all the way, they’ll get the fright of their life.
An alter.
What you have a hidden alter to is your business, but the ancient Egyptians and H. P. Lovecraft had some interesting ideas. Of course, the doors will have to open automatically when you remove the relevant book, or maybe a candle stick.
edit; missed post 33.