For starters, how about ...
Bwlch y Plwm - easy pull-though trip.
Pant y Wrach - short SRT trip, near to and could be combined with a Bwlch y Plwm trip.
Catherine and Jane Consols - interesting pumping system remains, chest-deep water in the deep adit and the possibility of an SRT through trip.
Brynfellin (near Beddgelert) - deepish SRT trip (not one I've been in).
Gilfach, Cwm Pennant - another deepish SRT trip (again not done any SRT there - hmm ... is a pattern emerging?)
Drws y Coed - more SRT (yes, there is a pattern)
Prince Edward gold mine - easy walk-in mine with a climb out through the stope.
... and there's more metal mines, at least one of which is very pretty and another that is very interesting.
If you say what sort of thing you're looking for we can probably find something to suit.
Dave