Advice on utilizing insulated garage

[John Carbone]

Have a 4 bed sleeps 9 with an attached garage that is already fully insulated with electric/windows/and two entrance doors. When you are in the garage it has the same walls as the main living areas, so apart from the concrete floors you couldn’t tell it’s a garage. I’m getting no real use out of it in the rental. I have a spot I can add a mini split for heating/cooling. 

Undergoing a renovation to remove the garage door but I can't decide what the best use of this space will be for a STR. I already have a small game room with 2 arcade games.

What will be my best ROI? The place is only a few years old so everything is pretty well updated.