Good choice. Wales is very underrated as a destination.
Some recommendations in North Wales:
Castles: Beaumaris, Caernarfon, Conwy, Harlech
Scenery: Snowdonia (the A4086 between Llanberis and Capel Curig is one of the best drives in the world - no need to go to New Zealand)
Steam trains: Llanberis, Welsh Highland, Lllangollen, Ffestiniog
Victorian seaside resort with pier: Llandudno
Towns & villages: Betws-y-Coed (can be very busy), Ruthin, Conwy, Beddgelert
Ice-cream: Glaslyn in Beddgelert.
Just inside England, Chester Zoo is a good choice - the bat house is is an experience and there is a big meerkat enclosure which the children will love. If you already have membership of Dublin Zoo & Fota, you get in free - no need to queue. Chester city is worth visiting too for its city wall and half-timbered buildings.