If you are visiting the mainland of Greece, the best time is April through November when it is warm and less rain. If you are also thinking of visiting islands such as Santorini, summer will be the best time, not only because there is almost no rain, but also because there will be many more tourism activities such as wine tasting than winter time. Be sure to check the weather forecast on weather.com when you are planning for the trip.
Transportation: Uber is available in big cities such as Athens in mainland Greece. Most people will rent a car or take a bus when they are visiting islands such as Santorini. Island airports such as JTR are usually small and have limited supply of rental cars, especially the automatic shift ones. Sixt, Avis, Budget, and other well-known car rental services are available in most airports.
Stays: Booking.com and Airbnb are popular vacation stay booking platforms for people who are looking for vacation rental homes in Greece. If you are looking for the best rate, GreekHolidayRentals is a local website to help you compare vacation homes or hotels rates from multiple sources.
Visas: For US citizen travelers, visas are not required to enter Greece. For non-US citizen travelers, Schengen visas might be required. When planning for your trip, check the official Greece travel advisory posted by the US Department of State. If you are in emergency situations or need local assistance, please contact the US Embassy in Greece for help.