I would like to visit San Miguel de los Banos in February but, so far, I haven't found any public transportation that could take me there from Cienfuegos or Santa Clara. Is there any way to get there if you don't rent a car in Cuba?
Go to the nearest Astro Bus station (the inter-provincial buses for cubans) and you are very likely to find a private taxi waiting for desperate customers nearby. These taxis are usually shared with other customers, but since you are a tourist they will probably try to keep you alone and charge you a different (higher) price.
By the way Astro Buses are meant to be used by Cubans only, but from time to time they have allowed tourists to get in. I am not sure which situation you will encounter on your arrival, but taxis I am sure will be available, and it will always be way cheaper than renting a car.
There are many ways to get to San Miguel. There are bus, train available frequently, you can always rent a car or hitchhike on your way. Cubans love gringos so there is fairly a feasible chance available.