
Off of the top of my head, Dawlish Warren is the only one I can recommend. It is restricted at this time of year, but you can access about the first third of the beach. Easy to find and park. Dawlish is really lovely but I guess it depends if you want to frolic on the beach or go for a long walk! There is another beach you can walk to beyond the small fair ground from there which isn't sand and which runs near a railway line. So you could do both.
Two others I know of but have never been to nor do I know much about are Shelley beach in exmouth and Jacobs ladder beach in sidmouth. I think Shelley is dog access all year. Google those three and see what you think! All around 25 mins-ish away. Wrong bit of Devon for me but maybe someone here will know better :)
Edit: Just noticed you said fairly quiet- doubt Dawlish will be quiet :-/
Jess