I’d look around Dolgellau / Mawddach estuary. Easy to get to CyB, Mach etc and loads of natural trails on the doorstep (Cadair Idris included). Also some lovely gentler trails and proximity to nice beaches etc for the rest of the family. Decent pubs nearby and Barmouth is just down the road for a candyfloss / arcade / fairground fix.
We camp here most years, but they also have some holiday cottages. Stunning location and highly, recommended.
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Further north then Beddgelert is also a good place to base yourself. Again great scenery, good mix of natural trails (including Snowdon). Not too far from the coast and the Welsh Highland Railway is lovely for kids (and adults!).