You cant depend on decent weather so you have to go out in it. Its Wales after all!
Caernarfon is pretty grim. Apart from the castle, its got nothing. Unsure if its open as its being renovated.
There’s Yr Hwylfan -soft play in a church next to Asda.
You could drive to Glynllifon - 15 mins away for a woodland walk.
Dinas Dinlle is a nice beach.
Llanberis for the day.
Betws y Coed.
Llandudno is nice. Shops, promenade, chips, ice cream and the pier (if its open).
Portmeirion
Are you bringing bikes? Great cycle path in town.
Fron Goch garden centre is nice for a meal and a mooch round their very expensive shops. Unsure if their Christmas site is open.
Y Galeri cinema is good snd very reasonable, but you must book online.
Wetherspoons in town is ok for a cheap meal. The Black Boy inn is really nuce and full of history.