Aberllyn Lead/Zinc mine is an excellent, all be it crumbly, trip near to Betws y Coed. There is a good SRT route down through the mine that gives the explorer a good workout whist also offering some opportunity to practice the proper vertical stuff that many of the angled slabby slate mine routes lack. Below is a topo that I drew up following a recent visit because there was only limited info about the SRT route available online.
Always remember that it is your responsibility to inspect any anchors and fixed rigging before choosing to use it. Also, there are many false floors in this mine and it really is only a place for people who know what they are doing and how to spot hazards in this environment. Check the access status before visiting.