We stayed this summer in Arzachena at Citti Hotel - a family run business. Altough we've read mixed reviewed, we've booked three nights and now I can say that it was a good decission. Che bella sorpresa! we've found warm people who welcome us with a smile, moreover make us fill our business is very appreciated. The room was cool and quiet, exemplarily clean with a large bathroom. The breakfast was delicious, my only recommendation would be if they can offer it a bit earlier, 7:00 am would be excellent, especially for the quest who wants to have a good start of the day and accomplich a lot in this beautiful area.
I found Citti hotel as an excellent home base for discovery Costa Smeralda, which is only 15 minutes away by driving.
Don't miss Spiaggia Liscia Ruia, Spiaggia Rommazinno, Capriccioli and Spiaggia del Principe. My favorite was the last one mentioned.
Other nice places to visit while you're in Arzachena would be: Coddu Ecchju - for a nice taste of "tombe dei giganti" in Sardinia, also Olivastri Millenari - some of the oldest trees in Europe and maybe in the world (it is quite impressive to see a 3,800 years old olive tree, measuring about 20m in circumference).
Enjoy a beautiful time there!