Without a car, serious transportation on these islands is not possible. In the Alghero airport there are several rent offices. The best conditions were in AVIS. Taking into consideration the serious discount, that was made for me, rent of the diesel Ford C-Max for 10 days cost only 246 euro (!) with standard insurance and opportunity to visit France by ferry. I have rented a car via credit card beforehand in USA with help from Booking Group company (www.economybookings.com).
It is necessary to be attentive to the working time of the rental service – usually it's from 8:00 till 23:00. That is why there can be some problems with late or early flights. Hertz works till 24:00 in summer, so despite of short flight delay, I have managed to take the car at 23:45. Returned at 6:00 thru the drop box. By the way, filling stations at night work in self-service mode, so do get some cash in order to fill your tank, before turning the car in.
Impressions, useful links and advice.
I used guide-book Polyglot thru Corsica and Sardinia. In my point of view there is all the information you will need, all the main objects are included. Plus really helps to use Google maps. I didn't feel any special cooking (besides the fish soup in Corsica), 80% of the customers in restaurants are eating pizzas with beer. I thought that level of service was a bit lower than in Spain.
Souvenirs are standard – vine, almond, macaroni, coffee, cork work. Plastic cards are accepted almost everywhere. The sea, weather and nature left me with good memories, although you won't find in Corsica or Sardinia nothing over the top (beach, ruins e.t.c.).