This is a terrifying power, I gotta say. Like a dude could carry an entire armory around and deploy it Looney-toons-style wherever he wanted.
I would probably just put a bunch of shelves in it and use it as a personal EDC gear storage space. 3m^3 is more than enough space to store pretty much anything you might need in a jiffy.
There's also apart of me that thinks putting a bed in there would mean not having to bring a date back to your place or to a hotel - but being real I'd probably never use it if I set it up like that.