answer:What are your hobbies? Do you enjoy video games? Classic films? Making music? Sculpting, painting, or making other forms of art? Yoga/pilates? A spare room could be used for any of those things. Maybe buy some shelves and fill it with books, throw pillows and comfy reading chairs (my personal choice). A spare pull-out bed or futon if you foresee any practical use for it. I couldn’t really understand the floor plan – where does the spiral staircase lead to? How much natural light does the room get?