When you visit Mykonos, you should rent a car for at least a day. The island is beautiful and it is nice to get away from the crowds for a while and explore the villages and the sandy beaches.
You can rent cars through your hotel. If you are going to fly in to the island, there are a bunch of rental offices at the airport (including some of the major rental companies.)
If you're there in July then A/C is a MUST. The jeeps don't have A/C, but that will not be an issue for you in September. The jeep is a great option. It is really cool driving around in an open air vehicle!
** Note: The original information for this thread came from the following forum thread:
