We fly in late at night so i was thinking about renting a car the next morning, sooo ar there any reputable local reputable car rentals in the city?
I haven't been to Cagliari yet, but have been doing some research. A place that seems (if you can believe Trip Advisor ETC reports) is CarBusTec, which isn't far from the ferry port, and only a little bit farther from the cruise port. I think there might be a thread on car rental or car hire in Cagliari as a "sticky" on Trip Advisor, and I found it to be helpful. They mentioned rental places that had been mentioned here (in negative comments) and gave suggestions as to how to rent a car - check everything, inside and out, be careful of restricted areas, which (apparently) may not be well marked for tourists who don't understand the language!), etc. There's also CIA Rental near CarBusTec, and an Avis place, but I don't know if those places are good or not. If you want something that opens early, it sounds like renting from the airport is best. There's a bus or train that goes there, or a somewhat expensive cab is an option.
But I haven't ever been to Cagliari or rented there, so can't really recommend anything. I'm hoping CarBusTec will be as good as their reviews seem to indicate. Certainly the prices seem reasonable, without being so low that they're suspect. They also rent scooters, but they're not all that less expensive than the cars.