What to do with 3rd Flr of a Duplex?

[Carl W.]

I'm purchasing a duplex with a 3rd floor that is mindblowing. Very spacious! I understand that I legally cannot rent it out as a 3rd apartment because it's a legal 2-Family, which is probably why it was never fully finished. But what should I do with all of that space? There's no way I'd allow it to just sit empty like it is currently. I'm looking for ideas of what to do with the space that could possibly net me some additional income. The plan is to fit the space with a full BA, BR, mini kitchenette, and a large open living/dining space. 

An Airbnb is one idea that I'm tossing around. The second idea is renting it as a long-term "room" that is an extension of unit 2. Another idea I had was moving into it myself and then renting out the rooms on the 2nd floor. It would still be my apartment, I'd just live above it mainly. Any other ideas of things I could do to monetize the space without getting into any legal issues would be appreciated. At the end of the day, even if none of these ideas pan out, I'd still have an epic 3rd floor for family and friends when they visit.