It would probably be best to learn some basic Welsh as you might encounter people who only speak Welsh - this is however unlikely, as the population of Welsh monoglots is in the low hundreds, most of which are elderly. Plenty of inhabitants speak only English - in towns such as Tywyn more residents consider themselves 'only English' as opposed to 'only Welsh' according to the 2011 census. Those unfamiliar with Welsh may struggle to read placenames, especially for those smaller areas without English names.
Plas yn Rhiw - beautiful, though not large, National Trust property east of Aberdaron
Penrhyn Castle National Trust - a mock-Norman edifice of the 19th century. Can be considered well over the top in the desire to impress but the kitchen quarters and the walled garden are particularly to be recommended. 
The Spinnies Tow splendidly situated bird hides administered by the North Wales Wildlife Trust. Birds frequently seen include little egrets and kingfishers. Near the coast immediately east of Penrhyn Castle estate.
Parc Glynllifon Lord Newbrough's estate south of Caernarfon - the arboretum and a modern amphitheatre are particularly worth seeing.
Ouzo and Olive, 14 hole in the wall street caernarfon (by the castle in Caernarfon). 6.30pm-9.00pm. This is a very nice Greek restaurant that is great for tourists. Very peacefull and relaxing. Open all year round. edit
Pale Hall, Palé Estate, Llandderfel, nr Bala, Gwynedd, Wales, LL23 7PS. UK, ☎ 01678 530285, . Palé Hall is a privately owned luxurious Victorian Country House Hotel near Bala in North Wales, set amongst 16 acres of gardens on the edge of Snowdonia National Park. Palé is a delightful discovery for discerning visitors - a lovingly restored Victorian country manor set in refreshingly tranquil and beautiful surroundings.edit