In reply to Flying Spaghetti Monster: Only really know about Cagliari and Cala Galone. Google is friend sir. Check the car hire sites and check like for like. been twice and used different firms, no difference apart from finding the cheapest each time. Its a tiring drive from Cagliari to Cala Gonone after a flight so a have decent car, even if you rarely use it after!
Accommodation is plentiful in Cala, so just google what suits you. Plenty of online booking opportunities. Some look cheap but charge for cleaning, linen etc. We ended using a lot of heating gas in Easter 2011 as it was bloody cold and wet a lot of the time!