1. Paint a matte muted color.
2. Remove spinning blade of mangled body parts, replace with something like a light bar or maybe sconce. I’d want it to shine the light upward and be partially shielded. A light bar is less of a target for head bonks.

Everything else depends on what you want to use it for.

If for storage, I’d put in some shelves for now with the goal of widening the entrance and having shelving slide out on rails in the future. Depending on what it looks like outside the room.

Otherwise this could be a good playroom for animals or children, secret reading nook, honestly as a kid I would have LOVED a bedroom like this. Decor for those uses obviously changes wildly but boils down to mostly paint and maybe some molding.