I have been a few years ago by ferry from the mainland of Italy. I am sorry to say there was nothing very memorable that we saw or did to the extent I cannot make any recommendations. I would be inclined to go somewhere on Italian mainland for your holiday and take the ferry across. That way you can spend some time without feeling you have to stay there for the whole of your holiday. We usually went to Tirrenia for the beach and Pisa for things like the inevitable leaning tower and the much better exhibition of ancient ships they came across when excavating at the railway station.
Thanks also, grasscarp. Just had an IM from an Italian friend who says much the same thing. Obviously, it looks better in a magazine article than it is in real life :-)
Never mind, at least it's given us the incentive to have a holiday, even if we end up somewhere completely different!
Sardinia is gorgeous..............I was once in the loo on a boat trip round Sardinia (did I say Sardinia was beautiful) when the boat went into a cave..........I had to sit there for 10 minutes as I couldn't see where the loo roll was.