Happy holidays and thank you so much for your question. It sounds like you have a great property. The fair rent does depend on a variety of factors including the local market, size of the space, proximity to attractions etc. I would speak to a local Realtor who specializes in vacation property rentals and get an idea of how to price the rental. It’s also important to get everything in writing even though they are already your tenant. A commercial real estate attorney should be able to help you draw up contracts.
Best of luck.