We stayed at Mount Edcumbe for a week and it was great. The estate was good for general wandering, has a small beach. Also has Segway and bike hire, and a massive frisbee golf course.
Big sandy beach at Whitsands, surf lessons I think too, and coasteering. Think you can do scuba diving out of Plymouth, and boat trips.
From the estate you can get the foot ferry to Plymouth, all sorts to see there.
An hour from the Eden project if you fancy that.
Depends what DC like. Our teenager is happy to go off for a walk alone, to sit and draw for a couple hours, and likes swimming, and museums, and will come for a couple of hours country ramble. But there's lots of other activities in the area, we just didn't need them.
The Rame peninsula is beautiful. We had great food too, at Maker Heights there's an ex-River Cottage chef so fabulous food, and a local Cornish pasty place, and artisan pizza in the local fishing village.
It was sunny the whole time but if it had rained, as I said lots to visit in Plymouth.