Mike, do you know the Bethesda woods? Probably not worth a trip in itself (I live across the road). DH trails and jumps up in Myndd Llandygai much better, and more chance of finding the trails! Snowdon is looking good. I walked the lower parts today and am mulling over a morning reccy on the bike tomorrow (no wind at all today).
OP, if you don’t know North Wales and don’t want to ride trail centres, go and do Snowdon. Conwy Mountain also another simple up and down route. Otherwise, for waymarked, I recommended Penmachno.