The Wheal Martyn China Clay Museum and Country Park (to give it its full title) was, when I visited in 1997 before the £1m revamp in 2005, a perfect little museum.
Based in the old Wheal Martyn and Gomm clay works, I particularly liked the way enough of the site has been restored to show how it used to work, but enough had been left overgrown to give a sense of discovering an abandoned site.
There is an Historic Trail past the largest working water wheel in Cornwall 10.7m or 35ft) and other artifacts, to a viewing platform at the top of the site where you can look down into the, still working, Wheal Martyn clay pit and see the water cannon (known as monitors) used to wash the china clay out of the ground.
There is also a Nature Trail, an Audio-visual presentation and all the usual visitor facilities. Excellent.
Check the Wheal Martyn China Clay Museum's website for opening times etc.