Dros y Dwr, Barmouth
There are great walks and hikes around Barmouth (for all levels of skill and experience) which take you through some of the most ruggedly beautiful scenery in North Wales - many you can start from outside the front door (so enjoy car-free days). There's good cycling too!
Mawddach Trail Ardudwy Way Mawddach Way Coed y Brenin
For sun-worshippers, there are many miles of beach and a short drive will get you to Wales' only 'official' naturist beach if you want an all-over tan.
Castles - of course, at Harlech and Criccieth, and the lesser known gem, Castell y Bere in the Dysynni Valley.
Quirky sights - try Portmeirion Italianate village.
And the Steam Train - through July and August travel the Cambrian Coast line by steam!