It's fairly easy to get around the island without a car! In fact, the best way to travel across Sardinia without a car is the bus. The trains are known to be unreliable and have odd schedules, but they're cheap. Other great options include renting a motorbike or hiring a car service. The ferry will get you to the smaller islands, where you can easily walk to more isolated beaches. If you can manage it, I recommend renting a car in Sardinia. You'll have more freedom to explore more of the island and reach more secluded beaches. We always use this site to rent cars in Italy. Be sure you have your International Driving Permit before going to Italy. It's cheap and easy to get, and it's the law.