Nice idea Greg.
Some info on a couple of your cards ...
The slate mine between Crafnant and Geirionydd is a number of entrances stacked up the hillside. Most of the trip is walking up to it and the exploration won't take very long (some nice pretties in there).
The "lead mine" running away from Geirionydd is a series of adits (possibly 8?) none of which are very long but some of which are quite pretty. If the stream water level is low it's great fun to combine "canyon walking" (easy version) with wandering in the numerous mines. We did it once and had a great time splashing around int he water too.
.... Of course, there is a video lol
Ian