I'd make mine into my "lady cave." I'd decorate it with sparkly textiles, lacquered surfaces, a tufted velvet setee, a chandelier, and art I love. There'd be a whole wall of books, and I'd put a TV somewhere for lots of shame-free, uninterrupted E! viewing. And of course, I'd have to include a fully stocked bar somewhere. Kind of like this:
What would you do with an extra room?