Renting a car is going to be the best value in most cases. While cars can get pricy, especially in high season, a car provides comfort, security and air conditioning. A number of the local rental car companies do also offer free parking in Mykonos Old Town, a much coveted perk, since parking is often expensive or impossible. We ended up renting a car from Anemos Mykonos (linked here). While you can certainly rent a car from one of the international providers like Avis, we found their pricing to be significantly higher and the local companies will come to you, whether that’s the airport, your villa or your hotel for no extra charge.
Car rentals in high season will run you somewhere between 50-120 USD per day, depending on the car. We rented a Fiat 500 convertible for about 70 USD per day for three days. It ended up being the right decision for us because the weather was over 100 degrees traveling with all our camera gear, it seemed much safer than an open-air UTV and we could have the top down when we weren’t dying in the heat.