I lived in Athens for over a year all told. Hit the Plaka area of town if your hotel isn't there, you can get to it easily on the metro. Santorini is probably the most beautiful island of the lot, you chose well. You can go out swimming and there are spots where the volcano is pumping up heated water from the sea bed, it's nice and relaxed. Whatever you do don't take the ferry to Ios, it's only a couple of hours away but it's not a honeymoon place. Go to the Sunset Taverna in Santorini, Costa Yiannis in Athens near the National Museum if it's still there, peace be upon it. Also try the row of seafood restaurants in Glyfada beside the harbour if you fancy a trip out of Athens, it's only a short taxi ride out of town. Fuck me, happy memories.