Some ideas:
A mine in the Stwlan area. Long walk up, wet feet, nice photo opportunity;
Betws Garmon / Ystrad / Garreg Fawr Iron Mines.
These are the series of workings on the hillside seen from the road between Waunfawr and Betws Garmon. I've driven past plenty of times but never visited;
Nant y Gadwen Manganese Mine, Llyn Peninsula. A fair old drive for most. Not especially extensive, but interesting and a nice area to visit. Possibility of new ground by boating across a flooded chamber to a passage visible at the end. Getting in and out of a boat could be tricky as there's no "beach".
Any other suggestions or votes for the above? :flowers:
"The true crimefighter always carries everything he needs in his utility belt, Robin"