Do we need to explain?! Perhaps the most iconic monuments that exist today, dedicated to the Goddess of Athena (protector of the ancient city of Athens). If you don't do a guided tour to hear the rich history behind these architectural feats, do go to the museum where they have videos and presentations on the history and preservation.
If you have the time, visit the monasteries of Meteora ~3.5 hours from Athens. These 11th century monasteries sit perched atop breathtaking rock formations. These monasteries make up the second most important monastic community in Greece, after Mount Athos in Halkidiki.