We stumbled upon Boyden Cavern on our way out to see the Roaring River Falls in King's Canyon (Highway 180), and it was a pleasant surprise. Guided tours are available several times a day, and is only $13 for an adult. Our tour guide (sorry, can't remember his name) was super informative and enthusiastic. I learned quite a lot from the tour guide about caves and the life that exists in it.
I also learned how horribly out of shape I am. We were warned that the walk up to the cavern was quite steep. This is an understatement. It is a MEGA steep walk up to the actual mouth of the cavern.
According to our guide, Boyden Cavern is a more intimate look at caves in comparison to Crystal Cave. I can't advise if this is correct since we opted not to visit Crystal Cave after touring Boyden Cavern.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.