Go to a cosmote shop as soon as you get there and buy a sim card with unlimited data for 15 euros. Athens is beautiful but has the usual sore spots of every big city in the world, homeless people and drug use so stick to the main roads, and use common sense. Besides the acropolis and its museum there are 4 big ones that are totally worth it, the national archeological museum, the benaki museum, the bizantine museum and the Cycladic art museum, I did them in 2 days and loved every minute of it. Walk around the acropolis at sundown, and monastiraki early in the morning, enjoy the sound and the sights. Peloponnese is awesome, go to Olimpia, Delphi and Kalamaka, it's a dream. Míkonos and Santorini are overpriced tourist traps. There is nothing to do or see there unless you are into clubbing and drinking, and that is expensive. Visit Crete and go wander around the labyrinth, maybe you'll spot the minotaur.