The area around the spectacular Golfo di Orosei, with its limestone cliffs and secluded coves, is a must see and you can pick up various boat trips from Cala Ganone. There is some excellent diving in this area as well and there are several schools that will hire out the requisite equipment. A little further up the coast at Santa Maria Navarrese there are other diving opportunities that are well worth exploring.
You’ll find abundant opportunities to trek in the mountains across central Sardinia and much of the time it pays to simply study a map and plan your own route (though of course be sensible). If you’re looking for something particularly impressive you could do a lot worse than the so-called Grand Canyon of Europe at Gola Su Gorruppu. This mighty gorge, purportedly Europe’s deepest, can be reached from the town of Dorgali.
The Costa Verde coast is at times a wild place and can provide some great conditions for windsurfing and surfing but as high seas can develop rapidly some caution is necessary. The beaches of Scivu and Piscinas are both renowned for their surf and both are easily accessed from the town of Arbus.