For this itinerary, transportation between cities can be made via rental car or bus. The travel time varies from 2-5 hours, depending on the destination. Cost is approximately €20 for buses and €60 for car rental.
If you have more time, consider visiting the islands of Mykonos or Crete. If you have less time, consider removing a city from the itinerary in order to have more time in each destination. Remember to dress comfortably and bring sunscreen, as you will be doing a lot of walking and exploring under the sun.