Many, many, many family holidays on Anglesey with my cousins. In fact, we were just in Betws-y-Coed in May for one of their weddings. It is absolutely gorgeous, if you like wet, verdant valleys, and slate mountains.
j/k, the beaches are great too. If you have wheels, its worth going to Bangor to see the Menai Bridge and the Narrows, and Red Wharf Bay on Anglesey is worth a visit too. At low tide, the whole bay is one giant beach. Just don't get caught out in the middle when the tide starts to come in though. You'll get wet in a hurry.
Bleanau Ffestiniog is worth a visit too, just because its fun to say.