Sardinia best beaches.

Hello,

We are a couple going to Sardinia in August for 15 days. We are landing in Olibia, and we are looking for a peaceful, relaxed trip in amazing beaches, but we dont know where is the best place to stay? Should we stay around Olibia or travel south? Where is the most beautiful quiet beaches in the island? We will rent a car and we are willing to move around, but not too much. The most important is to have a base in a nice place, with some life and restaurante with wonderful beaches around. Please any suggestion is very much appreciate:

Thanks, Sujaya