If you're going to North Wales, a trip up the Ffestiniog Railway to Blaenau Ffestiniog (take these spellings with a pinch of salt  ) is a must but make sure you check the time of the last train back or you'll find yourself waiting for a bus like we did!

A little way out of Blaenau, there is a slate mine which is well worth a visit. It even has a fernicular and an underground train ride.

And I'll second that pub in Capel Curig. We stayed in a hostel there call something like Tal-y-Bont or maybe Tan-y-Bont. Something like that anyway.