It is possible to rent a car from SIXT one way between stations on Crete. It is not allowed to rent between stations on the Greek islands and the mainland.
You will need a valid driver’s license, plus an ID or passport. Those with licenses from outside the EU will also need an international driving permit.
The minimum age for SIXT car rental in Greece is 21, with a driver’s license held for at least 1 year. Some car categories have a higher age requirement, and an underage driver fee applies to those under 23.