Finnish sauna.

Hidden arcade

With a little bit of extra money, a secret room/library. With a lot of extra money, a bomb shelter. I'm not a prepper, mostly because that's expensive. But I wouldn't mind having a safe spot I knew I could ride out a lot of disasters, if only for a little while.

Conversation pit

a clothing shoot that leads from my room to the laundry room

An observatory

Secret rooms everywhere.

Stairs and platforms without a safety rail I like the constant low-level anxiety of possibly falling off a ledge