Barmouth/Snowdon/Caernarfon is quite a large area! Themodernantiquarian, linked by Leon, is an excellent resource for stone circles and the like. He is right about Anglesey, it has more than its share of antiquities. It also has some beautiful beaches, impressive cliffs and mining ruins.
Back on the mainland I would suggest Lleyn as a good area for photography. Nefyn is particularly pretty; Abersoch has lots of posh boats and chalets or cottages of the well-heeled, while Penllyn (the far end) is thinly populated and more 'wild'.
Further South there are more terrific beaches around Harlech, and the Porthmadog/Penrhyndeudraeth area boasts two photogenic estuaries. I remember a lovely shot of the Dwyryd Estuary by Charlie Waite, it suprised me more because I thought I knew the area. However, you have to go hunting for that view (this photo is by Dave Newbould). The old railway line near Beddgelert is an easy stroll and the river Glaslyn and surrounding areas are very picturesque.
Barmouth and the Mawddach estuary is a fertile area for photography. There are great views above Barmouth and from the Precipice Walk at the top of the estuary, and the Cregennan Lakes above Arthog are worth a look. The old railway line on the South side is perfect for effortless walking or cycling along the length of the estuary.
You are spoilt for choice in this part of the world, though naturally it's very popular in the Summer months. For some inspiration you could browse Dave Newbould's website.