We just did almost this exact trip. We also stopped on the Amalfi Coast and Ephesus (which I"m sorry to tell you was our favorite port on the trip), and we didn't go to Constantinople. Other than that, it was exactly the same, including a few days in Rome before and Athens after.
What is your Sicily stop? We went to the town of Messina which was really a beautiful little town. I actually liked it better than the Amalfi Coast towns we visited.
Mykonos and Santorini are both beautiful, but honestly after we walked around a little, found a bite to eat and took some photos, that was plenty. They are small islands, and very similar, but the towns on Santorini are at the top of the mountain, and in Mykonos they are by the sea.
Athens was a pleasant surprise. My mother didn't like Athens and I had low expectations, but we really enjoyed it. I think she stayed in the business district which is indeed a little icky.. but the area around the Acropolis is beautiful, walkable, lots to see and do. You'll love it.
Happy to answer any specific questions if you have any.