Most British travellers know Spain as a beach, and miss much of the rest. Andalucía has the Alhambra, the cathedral and royal palace of Seville and the Mezquita in Córdoba. The Basque Country takes its food very seriously, and Mallorca has mountain villages and coves that feel nothing like the resort towns.
We plan Spain around cities and countryside in pairs: Seville with a finca in the hills near Ronda, San Sebastián with a Rioja wine estate, Barcelona with the Empordà coast. High-speed trains between Madrid, Seville, Córdoba and Barcelona keep the transfers short.
The honest caveat is inland summer heat. Seville and Córdoba are often close to 40°C in July and August, and the south of Spain sensibly slows down in the afternoons. Spring and autumn are far better for cities; save high summer for the north coast and the islands.