best time to go weather-wise is june to sept. i know u mentioned staying at some "rustic" places, but i suggest u get the SPG Amex since they have an abundance of great Spanish hotels.
San Sebastian - Hotel Maria Cristina
Elciego - Marques de Riscal
Barcelona - Le Meridien
Madrid - The Westin Palace
Seville - Hotel Alfonso XIII
Valencia - The Westin Valencia