If you have only gone to South Stack on Anglesey you have missed out on many other smashing places to Birdwatch. RAF Valley (Cetti's Warblers etc), Cemlyn Bay (Big Tern Colony), Fedw Fawr (Black Guillemot) Malltreath (Estuary Birds) Newborough (Woodland Birds) and that's just for starters.
I have spent many happy days on Anglesey and have also had holidays there. Apart from Anglesey, if you spend time on there, you have some excellent places to go on the Welsh mainland such as Llanfairfechan (looking across the straits to Anglesey) Aber Falls (a lovely walk up to a waterfall, I have had Nightingale here) then there is Snowdonia and here there is a place called Padarn old Slate mines) Ring Ouzels have been known to breed here and most of the woodland birds are here also such as Wood Warbler, Pied Flycatcher, Redstart etc, plus Peregrine are regular there.
If anyone else is thinking of spending a day on there give yourself plenty of time on there and if you want directions of places to go you only have to ask.
As an extra note, on Cemlyn Bay at the moment there is a bird that is getting a fair proportion of birders excited. It hasn't been firmly ID'd yet, and I'm not sure if it ever will, but there is a Tern that is coming on the pager that is being put down as a "probable Cayenne Tern"
Here is a link to what they should look like.
http://www.arthurgrosset.com/sabirds/cayennetern.html
John