I have had many happy holidays in Cornwall.
Photographically the greatest challenge, IMO, is where to point the camera first - the possibilities are seemingly endless.
However, I have unfortunately found that Devon and Cornwall are almost an entirely film free zone - so take plenty of film stock plus a healthy reserve (I find it amazing how fast I can get through it with the right light and subject material)
The Dartmoor Tors are worthy of an investigation if the light is interesting (sunshine & showers for me)
While I am not by nature a morning person, I do find the first few hours of daylight to be photographically the most rewarding, as I usually don't like people in my shots and the hoards don't start to appear until around 10:00