I would buy an SVR "any permitted" ticket from Cambridge to Bangor (£64 adult with FF railcard). This is a VT fare so no time restrictions to/from Euston. I don't think there are any restrictions to/from King's Cross either, but no doubt somebody on here will know. 10 minute wait at Chester for an ATW Bangor train if you are not on a through service. For some reason the VT fares do not extend beyond Cambridge.
Connections on XC via Nuneaton are hopeless and not guaranteed.
You would have to get a local bus Bangor to Caernarvon and then the WHR to Rhyd Ddu.
On a good day you can see the summit of Snowdon from Rhyd Ddu WHR station, and the Snowdon Ranger path passes there, but it is a long slog up the hill and not the most interesting route. You could get the Sherpa bus to Pen-y-Pass for the uphill stretch and come down the Ranger to do a circle.
In the good old days I did a day trip from Chester to Snowdon using train to Betws-y-Coed, bus to Pen-y-Pass, PYG track up, Llanberis path down, train back to Chester (in the days when there was still a summer service from Llanberis). Great fun.
Richard
Even better, buy the VT "via London" SVR to Bangor from Newmarket (same fare and conditions as Cambridge - £64.10+£18.45 with FF card).
The XC "Not London" fare from Bury may be a bit cheaper but has an 0930 start restriction both ways and has terrible connections whichever way you go.
