In reply to benmason:
All the single pitch crags within Snowdonia are pretty mediocre for groups in one way or another, although still very workable for introductory sessions. The usual options have already been mentioned.
However, one of the better single pitch venues in N Wales (especially the group is fairly active and have climbing shoes) is
Pot Hole Quarry (Three Springs) fairly close to Mold and the A55 so potentially a good venue on the first or last day of a trip.
Post edited at 11:41