I can recommend a downhill MTB run from Dolwyddelan to Betws Y Coed. Let the train take the strain uphill and then mountain bike downhill.
Scenic little trip - and quite okay for families. There is a very short section crossing the A470 where you have to take care, although you don't need to ride along that road, as there is a dedicated shared use path alongside it.
If you get the train times right, you could catch the next one up and do it again. Less than 5 quid for dad and son.