Right Martian, re the steam train in Caernarfon, \link{http://www.whr.co.uk/\here} is the website.
I told you that trains go as far as Rhyd Ddu,a couple of miles outside of Beddgelert (and foot of Snowdon)
I suggest you drive to Rhyd Ddu,park the car and get the train to Caernarfon,taking the train back to Rhyd Ddu a few hurs later.
The train also has a bike carrying facility,and you can hire bikes from a place near your campsite or from a place on the quay on Caernarfon (also tagalongs and baby cart thingies).
This is great too,because there is a fabulous cycle track which runs parallel to the train track so you could engineer some train/bike combo.
The best drive in the whole of North Wales is from Penygroes to Rhyd Ddu through Drws y Coed. You must do it,hardly anyone knows about it.
If you are doing a loose circle as I suggested before (eg Beddgelert,Cricieth,Porthmadog,Caernarfon,Beddgelert) then break off at pen y Groes and go down Dyffryn Nantlle (Nantlee valley) through Tal y Sarn (not so pretty but don't panic,it gets better)on to Nantlle,Dwrs y Coed and Rhyd Ddu then Beddgelert.
It is one of my favourite places in the whole world.Look out for the little chapel in Drws y Coed,then on the other side of the road,keep your eyes peeled for a huge boulder.
There is a sign on it (Welsh only) explaining that it rolled down the mountain and crushed the original chapel which was built on that sight.