Like others have said, all of the area is massively busy in the summer. Bosinney Bay near Tintagel is an amazing little cove. You gave to walk down a crazy winding path to get to it but it's well worth it.
Close to Rocky Valley and St Nectan's Glen, for exploring. There's enough walking around Tintagel and Boscastle for a good few days.
Edit: If Cornwall isn't already full...