Set up with a standing shower enclosure, across from that a stall with the toilet. Next to the stall, a simple sink and mirror (it probably wouldn't be hard to get a wall mounted soap dispenser too). Next to the shower is a couple sturdy towel hooks, and 1 or 2 generously wide & tall lockers. And of course, in the centre of the room is a simple, floor mounted, flat wooden bench.
Functionally, it still works as a bathroom, but aesthetically it'd give you a lot of fun options