Bibi R.
Yelp
Really loved the Slate Museum. Unfortunately, we were there in the off season, and the weather was a bit cold and wet, but the museum was fun anyway. It is mostly outdoors, so you may want to watch the weather if you would prefer not getting cold and wet.
The museum is set on the bottom of the slate mine, where the quarrymen used to have their lunch breaks, make tools, split slate, etc. It's very impressive and they show a short movie on the history in the beginning of the tour (on the hour). Unfortunately, we missed the splitting demonstration, as they were understaffed that day. But all the people working there were super nice and friendly to us, and they were more than willing to answer any questions and explain any process.
The giant waterwheel is very impressive as well. I would definitely recommend taking some slate with you (£1.75 per piece and they are available right in front of the museum shop), and I also bought a cute card made from sheep poo. Amazing! :-)