Muscat Royal Opera House, a very exquisite new theater, with white exterior walls and tiles that are very noble, and there are supporting commercial areas and high-end restaurants around it. If you are lucky, you can visit the performance rehearsal, but you are not allowed to take pictures.
I thought I could enter for free, but later a local brother reminded me to buy a ticket (at first he wanted us to give him cash, but later we insisted that he take us to the ticket office, 3 riyals per person)