Tips and Questions for Sardinia?

Hi all, my fiancee and I are visiting Castiadas (Southeast Sardinia, staying at La Villa del Re) in mid-September 2023. We are looking forward to just relaxing on the beach, but definitely want to get out and explore.

Any recommendations for hikes, excursions, towns nearby to visit, other points of interest, as well as dining would be very much appreciated! We are there for 5 days. As of now, we are thinking a day trip to Cagliari and/or Villasimus, probably some sort of boat/sailboat rental, and a hike or mountain biking excursion of some sort.

Any thoughts or recommendations would be very much appreciated!