you can due athens in one night man so dont put too much time on your itenarary there. One tour of the city and you can see it all.
I went to Santorini and to Mykonos and they one main peice of advice is that make sure and i mean doubly freaking sure that you get on a high speed fairy.
Once you get to the islands you will ber rocking, you really dont need much advice. However, pretty much the best beach in greece is one called Paradise wich is on Mykonos. I will attach some pictures of my trip man.
One more piece of advice is to have a travel agent plan everything. you can find them on the internet but i will see if i can find the one i used and give you a link. they planned everything. When i got off the plane there was a guy to pick me up to take to the hotel. The next morning a guy picked me up to go on a tour of athens. fairies and hotels were booked as well. basically i did not have to worry about a single thing not even a taxi ride.