The Mawddach estuary between Dolgellau and the sea could keep you occupied for a week! Seriously.... and a great time of year to be visiting.
There are some fabulous wooded ravines in the area, best in cloudy conditions; some great viewpoints eg Precipice Walk, New Precipice walk, Panorama walk ; the Cregennen lakes (fabulous at sunrise) , If you're up for a hill walk but not the full Cadair Idris, take the Pony Path up from the car park and when you reach the ridge, instead of turning left for Cadair, go right for Tyrrau Mawr. Amazing coastal views.......
I won't give any map refs because i find half the fun is researching the locations first.......