How far time/distance are you prepared to travel?
Underground only or surface walks/features?
2nd vote for Big Pit, add a visit to Blaenavon Ironworks museum (also free) as well at the same time.
Within an hour or so's travel you have the Forest of Dean, Cwmystwyth, Elan valley, numerous minor trials/mines of mid Wales and of course all the S Wales caves... Take your pick!