It is recommended to use ferries for long-distance transfers between islands. The estimated cost of each ferry ride is around €20-40 and takes around 2-3 hours per transfer. Within each city, walking is the best option for exploring the city's landmarks and hidden corners. For longer distances, taxis and public buses are available at reasonable rates.
If you have an extra day, consider visiting the stunning island of Crete, famous for its beaches and ancient history. Additionally, you can also explore the city of Nafplio, located near Athens, with many historic sites and beautiful views. If you have less time, consider removing a city or combining two cities into one day to make the best of your trip.