Piran is a very small, but nice harbour city. Our trip there was during mid-October, when the weather there was gradually cooling down, but still got a bit warmth.
In the morning before 12h, it was always foggy and felt a bit cold in an uncomfortable way, plus there was also a humid feeling with wind blowing into your skin. But after 12h the weather turned nice with beautiful sunshine and clear sky.
The tanning season on the beach was already over, leaving only some empty chairs on the sands. However, there were still a lot of people walking along the coastline enjoying the nice view of sunshine and sails.
A lot of small cafes are located along the coast so that you can enjoy a cute drink whenever you feel tired. And you can walk from Piran through Portoroz and back again to Piran, which was what we did.