Right off the bat, if you want a guaranteed guarantee, you look to the Canary Islands. Places like Lanzarote or Fuerteventura sit so far south, they practically wave at Africa.
In September, you’re looking at 27°C (81°F) during the day, and the water is still a lovely 23°C (73°F). It’s like the ocean forgot it’s supposed to cool down. You get reliable sun without the sweaty, crowded feeling of July.
Plus, the landscape is almost lunar — volcanic black sand and dramatic cliffs. It’s different, and that’s what makes it cool.