In reply to Mischap:
I'd suggest something using the Conwy Valley train line. Start at Dolwyddelan, go over Moel Siabod via the Daear Ddu ridge, then on over Carnedd y Cribau, Moel Meirch, Ysgafell Wen, Moel Druman, then down to the station in Ffestiniog. Or the same start and finish points, but go over Y Ro Wen and Moel Penamnen instead.
Or you could do something really long and ambitious, like Conwy to Betws y Coed over the edge of the Carneddau