I can recommend Cwmcuttan, lovely carp lakes.
Also C&CC site Bedgellert. Steam train has a stop in the campsite, south to Portmadoc or north to Caernarfon.
Not been to Jim's site yet but hear it's very peaceful.
Were booked in for 2 nights for the Beddgelert one after we leave Shell Island, but have no plans after that, just meandering our way back home. Then remembered the site in Cwmcutten but can't find any more info about it!