Athens: the place where so many things were born, such as philosophy, theatre, democracy, etc. The Acropolis and the slopes surrounding it are amazing. Just make sure you spend enough time here under the olive tees, wandering though the paths that lead to ruins and more ruins of amazing things you have read in history books. Apart form the acropolis there are a couple of great museums and some cute smaller churches. Athens enjoys good weather, so staying out, dining and wining is always a plus.
Insider Tip:Apart from the ruins and a couple of museum, there isn't much more big sightseeing to do, but there are a couple of excellent day trips you can do if you have one more day. It was super hot so suncream and hat are necessary.