@Francesca Olbia and the surroundings are fantastic, although I'm not a fan of Porto Cervo but the sea there is like heaven. I love Arzachena and Porto Rotondo.Anyway, it's a great idea to rent a car! Maybe your daughter in law and your son still know it but I suggest to made a trip to Tavolara Isle ( Olbia) with a boat or also Cala Molara and Cala Molarotto. Well, Cagliari is amazing. All that area is super. Villasimius,Castiadas. But there is a place very special: if they have time they can't miss to visit the Dune di Porto Pino-Sant'Annarresi (Giba).And the cities of Santa Margherita di Pula and Nora (fenician).
@MypinchofItaly
I shall definitely call them after lunch and tell them !!!
I appreciate very much all your recommendations.
Thank you so much ..
Have a lovely lovely day.
Ciao ..