In reply to drolex:
Start from Gerlan, follow the marked track running above the N bank of Afon Caseg, heading for Ffynnon Caseg. An intimate little cwm gives access to the shaly E ridge of Yr Elen. That way you get the sudden panorama of Crib Lem and crew at the summit.
Head back to Gerlan NW from Pen yr Ole Wen. The descent is a bit dull, but the views across the Nant Ffrancon from Braich Ty Du and further along past Ty'n y Maes are worth it.
Scrub all that if the clag's down.