Blaenau's main attraction takes you into the bowels of a Victorian slate mine. You descend the UK's steepest mining cable railway into the 1846 network of tunnels and caverns, while 'enhanced-reality technology' brings to life the harsh working conditions of the 19th-century miners – be prepared to duck and scramble around dark tunnels. There's also a tour of the quarry in a military truck (£20 per person). The first tour each day is half price; check the website for times and prebooking.
Llechwedd Slate Caverns
Top choice in Snowdonia National Park (Parc Cenedlaethol Eryri)