One of your first stops should be Coed y Brenin Visitor Centre. Here, you'll be able to find your bearings and get some great ideas before you set out exploring.
Situated in Dolgellau, this tourist information center is well worth including on your itinerary. But you'll find a great deal more to see in this area. Eryri National Park, Torrent Walk, and Cymer Abbey are within close reach of Coed y Brenin Visitor Centre.