Watercolour Cottage is in Lerryn. That's near Fowey, up the river. It's a small cottage, one of a row, painted white. It sleeps four, with two bedrooms and one bathroom. The rating is 3 Star, which is accurate. It's not big, but it's comfortable.
The village is basically a river, a bridge, and a few houses. The river's tidal, so you can paddle or mess about in a boat when it's up. There's a small shop and a pub, the Lerryn Inn. It does food and beer. You can bring one dog.
For walking, you can follow the river paths. The Lerryn Woodland is nice and shady. If you drive, you're about 15 minutes from Fowey for shops and boats. Polperro is about half an hour one way, Looe about half an hour the other. The Eden Project is a 25 minute drive inland.
Inside, it's got a woodburning stove, which is good for evenings. There's a washing machine, a cot if you need it, and WiFi. The TV works. Bedding is included. Parking is on the lane outside.
If you're looking for more ideas, the National Trust Cornwall page has good information on places nearby.