Refinish it and put a couple of rooms and tables in there for massage therapy or physio, chiropractors.
Off the top of my head though I think the massage therapy (or reiki, reflexology, etc.) route might be a better ROI because with chiro you might have some expensive renovating to do. But with RMT, etc, depending on the condition of the place you might be able to get a 'rustic spa' look with less initial capital.
See if there is a market of therapists out there who would rent out a room(s) from you, before you jump in obviously haha
But it's very common for massage therapists to rent out rooms from businesses that have the space with different rates for a 1-day rental, or a week at a time, or rent for a full month. This is a good deal for you because rent $ is upfront to you and it's up to the therapist to fill their rented time with clients, no risk to you.