Hellow all...
Back again planning for a five week stay in Italy starting early September. Of Alghero, Gagliari or Olbia which would offer 2-3 days of interesting sights, etc. without needing a car? We realize this part of the trip would not be slow travel and are wondering if 2-3 days there is worthwhile considering time and expense of getting there (plane or ferry). If we could stay longer, would we need a car to do Sardinia more justice?
Thanks for all ideas and suggestions.

The places you mention, Alghero, Cagliari and Olbia are all very different in terms of terrain, culture and "sights." Cagliari is the largest and most developed city on the island and you could spend a few days without a car. Alghero, Bosa and the western coast are certainly worth exploring. However, to do the island justice, you must see its coastal areas and for that, you will need a car. There is a train that runs through the interior,and a public bus system, but I wouldn't recommend depending on public transportation. If you go to the northeast coast, and plan on visiting the areas around Palau and the Costa Smeralda, you will most certainly need your own transportation. Car rentals are readily available, however, both at the airport and within reach of the ferry ports. If you only have a few days, I would choose one area and plan your activities accordingly. If not, you'll spend a lot of time just driving.

W/o a car, most definitely Cagliari. If you have a 5 week trip why only 2-3 days in Sardegna? I'd take a week, get a car, and enjoy the island. The beaches there are incredible, and in September there'd be few tourists and the water would still be very warm.

I forgot to mention that if you are considering renting a car, you have nothing to fear with the Sardinian road system. The main roads, especially along the coasts, are modern with great signage and are very easy to navigate.
And Rar is right about the water temperature in September...it is at its warmest and absolutely delightful.

I agree totally with Katherine and Rar.
Once you get there, you'll want to go exploring along the coast, and you do really need a car.
Roads are excellent, traffic isn't bad. September is divine for swimming, and you'd miss the crush then too. But if you are absolutely convinced that you don't want to rent a car, then opt for Alghero -- it's a lovely town to explore, with atmosphere, great restaurants and wonderful food, interesting alleyways to get lost in, and the public beach there, as far as public beaches in Italy go, is relatively uncrowded (in September) and clean. It's not like the sparkling coves of the Maddalena, but in September it's a decent beach. The public beach area is a bit far from the old district, where you'd probably want to stay, but there must be local buses going up and down along the lungomare all the time.

As for Cagliari, the public beach area there was pretty nice too. But I have only been there in September and June.

There's also a special train serving tourist areas, which is supposed to quite charming, which would allow you to visit places you might not be able to get to otherwise without a car It follows some of the track DH Lawrence followed as described in "Sea and Sardinia" I haven't got the link, but somebody else might know if it's still running. I' ll try to check later and post again