A car is useful for exploring Rhodes or Kos, or to hop between Patmos's many beaches. In Symi, with its few roads, a car is of little use, and you should opt for the vans that serve as the island buses, make use of the island taxis, or walk along the paths that connect most places on the island.
You may take a car to the Dodecanese on one of the large ferries that sail daily from Piraeus to Rhodes and less frequently to the smaller islands. The relatively small network of roads on Rhodes is well maintained and detailed maps are available; traffic is likely to be heavy only from Rhodes Town to Lindos. In Kos, a car makes it easy to skirt the coast and make stops at the many sandy beaches, though resorts are serviced by bus. In Patmos, a car or motorbike makes it easy to tour the island, while sights and outlying restaurants are easily reached by bus or taxi, and a few beaches can be reached by either bus or boat. Symi, which has only one road suitable for cars, is best explored on foot or by bus or boat. Expect to pay at least €30–€40 a day to rent a car on one of the islands.