The contributions the Greeks gave Western civilization

The Greeks are rightly seen as a founding influence on western civilization. Athenian participatory democracy was a form of government the West long sought to emulate. The tradition of criticism of that same system and of society that first began with the philosophy of Socrates is another contribution. Greek philosophy has been very influential; the pre-Socratics and their attempts at naturalistic explanations laid the foundation for scientific inquiry. The philosophical idealism of Plato has been a powerful influence throughout Western history as have the writings of his student Aristotle. Aristotle's ideas about biology weren't eclipsed in some cases until the early 19th century.
Beyond this there is the importance of all of Greek mythology and literature, which became canonical in the Western tradition and echoed time and time again in countless works of fiction and non-fiction. The Greek alphabet became the framework for the English alphabet and Greek stories became foundational for the West more generally. Though it was adapted to Roman purposes, Greek mythology and thought was embedded in the Roman empire and this is one of the ways it first spread. Greek writings were also preserved in the Islamic tradition and it is these Arabic translations that helped spawn a revival of classical ideas during the Renaissance.
In essence, one can argue that Western civilization is founded on the ideas of the ancient Greeks.