lots to do in that area, don't know ages of children so this will depend on kind of things you want to do.
llandudno.. on Great Orme is dry ski slope with toboggan run (son used to work there in summer hols from Uni)
Snowdon of course... you need LLanberis for the rail way up there and you can of course walk it!!
Also at Llanberis
Dinorwig , a water power electricity station in the side of mountain... you don't know its there but they do trips inside and we found it worth the visit, but not been for a few years now so may have changed.they do have a web site but cannot access it at the moment.
Padarn country park... slate museum and lots of walks, lakeside railway too.
Greenwood forest park
eco friendly park with a people powered roller coaster (son worked there too!!)
Anglesea sea zoo not been for a while (son scuba dived in the tanks!!)
Then you have lots of castles and walks all over the area. Conwy, Beaumaris on Anglesea,Caernarfon
Ffestiniog Railway Narrow gauge railway.A lovely trip, well worth visiting.
Portmerion Home of The Prisoner and an unexpected delight.
As you may guess love the area, visited lots of times and son went to Bangor Uni and still lives there so visited several times since. hence the reason for their location...
wish you hadn't asked now!!
More here worth a look
north west Wales north east wales