In reply to Russell Lovett:
Capo Testa is the spot for sculpted granite boulders by the sea. I know you said "North" but if you don't mind a bit of driving (it isn't a very big island) I'd also recommend the enormous abandoned mine workings around the south west corner. Anywhere between Iglesias and the charmingly named Buggeru is worth a look. There's plenty of good climbing there too.