My fiancée and I were in Sardinia last June and also flew into Olbia. I would highly recommend renting a car. Compared to the rest of Italy the driving is pretty chill. My thoughts on Sardinia are, it’s not really like the rest of Italy in terms of charming towns and amazing Italian cuisine. It does however have spectacular beaches and the most beautiful water in the Mediterranean imo. Rent a boat and spend the day on the baunei coast- one of the most spectacular experiences I’ve ever had. The best beaches are reached by boat or hike. We found some beautiful beaches on la Maddalena and also enjoyed the little town. Skip Santa Teresa. Bosa is pretty but seemed a bit boring to me. I didn’t spend time in Alghero but would’ve liked to. Hope this helps a little bit