Island hopping requires time it seems you don't have, with only 5 days give or take for Greece. The best advice I can offer is for you to fly to Athens. In the long run I don't think you'll pay more for airfare than for transport by ferry when it has to include transportation to one of the east-coast ferry ports (Bari, Brindisi, Venice, Ancona) and possibly an overnight in a hotel, plus the time it takes to get from the Greek port of Patras or Igoumenitsa (on Greece's west coast) to Athens (on the east coast). There is no ferry from Italy all the way to Athens. And then what will you do, sit in a chair for 32 hours or book a cabin that costs a lot more? What's more valuable in this situation, your time or your money? Choose a Greek island with an airport. Choose one that you can make same-day connections to on either Aegean Airlines or Olympic Air. Both fly from Venice, Rome and Milan to Athens but both don't serve all Greek islands with airports as some airports are too small for the Aegean Airlines fleet. All the islands you've mentioned will be crazy-busy in August. There won't be any Greek island, in fact, that will feel kicked-back in August, so you might as well choose one that offers you the widest variety of things to see and do, and then try to find a location suited more for relaxation than for partying.
My all-time favorite Greek island is Santorini. Oia is by far the most beautiful village. It's the one that's used for most of the travel brochures you see for Greece, the ones with the photos of the blue-domed chapels. Yes, it's crazy-busy in August. Yes, it can be VERY expensive. But it doesn't have to be. Depending on what your nightly room budget is you can find lots to choose from.
Use the filter system at http://www.booking.com to search for affordable accomodations on whichever island interests you. Base your choice on location, but this means doing some more research. If you do choose Santorini let me know what that budget will be, in EURO, and I'll help you find someplace to stay.
