Presumably you arent just on a trail centre tour? Definitely stay in Snowdonia. Loads of campsites around Beddgelert, Capel Curig and Betws. Cae Du is a good one with pubs near by and a stunning location near Snowdon. Book in advance though.
Mid Wales worth a trip too, couple of days riding around Rhayader would show you a different side of Wales.
As the guy says above, less to do for non outdoorsy types in N Wales than, say, the Lakes. There are some good beaches though, and places like Beaumaris and Portmierion. Plus various copper, slate mines and Electric Mountain of course….