It would be helpful to know what your facility’s space and design are, and whether you just want to rent an exercise room, for example, or also use of your equipment, of if you have unused meeting rooms, and whether you wish to rent during your own down times, or if you just have extra space that can be rented on a semi permanent basis.
Also remember to check with your insurance to make sure if you sublease to someone, particularly someone whose services are different than your own, there will not be an increase in insurance that will eat up any profits.
Depending on your layout and soundproofing if you have a private room you might be able to rent to a bodywork practitioner. Many who do massage or Reiki or things like that are independent, and even if they have a regular space it can be good advertising to take some clients at another space. And the good thing is if you do not offer that yourself there is no competition. You would need to determine going rate in your locality to determine space rental, and check with your insurance company.