To some extent, the best topic for a paper on the history of Spain is a topic that interests you. It will be somewhat difficult to write a paper that long on a subject that bores you. You may want to think about what sorts of topics would be fun for you.
Here are a few topics that could be interesting and could be sufficient for an 8-page paper.
- Muslims, Jews, and Spain. For part of its history, large parts of Spain were under Muslim rule. At some of these times, there were Christian and Muslim kingdoms existing side by side in what is now Spain. This made for a great deal of interaction between Christians and Muslims in Spain. There were also large numbers of Jews in Spain. You could do a paper on the interactions between people of the three faiths and how that affected Spain.
- The conquest of the New World. The Spanish were the first to have major holdings in the Americas. The Spanish conquistadors took huge areas of territory and obtained great wealth from those areas. You could do a paper on how Spain administered its territories in the New World and how those territories affected Spain.
- Spain and its relations with other European countries. During the 1600s and 1700s in particular, Spain was a major power in Europe. It ruled parts of Italy and what is now the Netherlands and Belgium. It was involved in many wars in Europe, partly due to the conflicts between Catholicism and Protestantism. It famously tried to invade England with the Spanish Armada. You could write a paper about how Spain was involved in European power politics during this time.
- The Franco dictatorship. Francisco Franco was a fascist dictator who ruled Spain from the late 1930s until the 1970s. You could write a paper on many aspects of his rule. You could write about how his side won the Spanish Civil War. You could write about what he had Spain do in WWII. You could write about how he ruled Spain and kept power for decades after the war.