@Baruch and I had similar questions - check out these threads we posted:
and
People had lots of suggestions and recommendations that may be helpful to you, too.
You may also be interested in the “after” threads:
and
Baruch also posted about a later trip here:
In short – Anglesey is a good place for speaking Welsh; I’d also recommend Caernarfon.
And if things work out for both you and them, try to visit @aran and his family if you’re near Caernarfon. It was lovely for me to be able to meet the people I had heard so often as voices.
Also, if you happen to like Rownd a Rownd, a trip to the film set in Menai Bridge might be interesting.