How about Dartmouth area? Nice sandy beaches, Exeter has a good leisure centre and other facilities, and Crealy adventure park is not too far away. North coast better if you prefer craggy coastal paths and body boarding. You'll need a car to get about.
Depends when you are going as to what accom you will find. if it is now in school hols then hotels, b and b and camping will do short breaks but most likely self catering cottages and caravans won't. Agree with suggestions for Paignton or Torquay but sort your place to stay before you go don't just turn up as they get busy in school hols