In my experience the weather in Spring (say May) or Autumn (say mid Sept to mid Oct) can vary between totally cloudless days to rather indifferent. For instance, Oct 8-10 last year was dire, with periods of extremely heavy rain and wind and the most mountainous seas I have ever seen in Plakias.
If you want to be guaranteed wall to wall sun then you need to go in high summer, but then it is too hot for many, can be extremely windy, and you will pay the highest prices.
Spring and Autumn are great for walking (assuming it isn't raining) and the sea is at a reasonable termperature for most.
It is a well established fact that Autumn is much busier in Plakias than Spring. I have never understood why (does anyone know?) as it doesn't apply to other parts of Greece I have been to. The best tavernas are full from about 8pm until late.
Mike