I visited the Geevor Tin Mine in Cornwall. There is an informative website (how to get there, opening hours, etc.) at http://www.geevor.com/

I took bus no 17 from Penzance. There are a couple of buildings one can visit and a very interesting underground tour to a mine tunnel - highly recommended.

All the mines in the area were family businesses, with just 10-20 employees. The Geevor mine is about 300 years old and was just recently discovered.

If you stay in Penzance, there are lots of other great places to visit in the area: St Ives, Minack theater, Scilly island, etc.