Cadiz is one of the oldest cities in Europe. It's location is quite unique, since it is surrounded almost completely by water (similar to Venice). James Bond used the city to resemble Cuba.
Jerez is known mainly for two things: 1) the bodegas for the finest Sherries and 2) the Equestrian School for ultimate horse back riding demonstrations. Add to this great weather, some flamenco and lots of orange trees and then you know something about Jerez de la Frontera.
Mallorca (aka Majorca) is a very well visited Spanish vacation island in the Mediterranean. Many Germans, Brits and other Europeans spend their vacation in one of the many resorts on one of the many beaches - or just in a Finca in the main land. Some even spend the whole winter there and enjoy the pleasant climate and atmosphere. Palma de Mallorca has excellent connections by air to many European cities.
Pamplona is best-known for its annual "running with the bulls"... Yes, these are real bulls and it is dangerous! Only the very sporty ones that have something to prove, should try this...
San Sebastian is a superb beach location just west of the Pyrenees with excellent food (tapas). It should be on everyone's itinerary.
The Canary Islands are politically part of Spain, but closer to Morocco. The islands are mainly volcanic with a very pleasant spring-like climate year-round.
Santiago de Compostela is the capital of Galicia and the end point of the pilgrimage "Camini de Santiago" and the apostel James should be buried here. The journey here is different from other journeys since people like to either walk or bike.
Segovia is located just north of Madrid and very well worth a day-trip. The main sights are the old city center with large Roman Aquaduct, the Cathedral and the Phantasy Castle by Alfonso II.