My wife and I took a day trip to visit Giglio Island while in Porto Santo Stefano. It is a very easy ferry ride from the dock in Porto Santo Stefano. Tickets round trip are 20 euros pp and the ferry ride is a beautiful slow ride that last about an hour. The ferry also transports cars if you want to take your rental. We chose to leave the rental behind.
The trip to Giglio is very scenic. As you pull in to the port you go right next to the Concordia Cruise Ship that ran a a ground back in May 2012. So close you feel like you can touch it...that was very neat to see.
The village itself is full of shops and restaurants. We had a great lunch and spent about three house exploring and taking photos. Our time was limited, so we did not visit the walled castle on the top of the island...I was told the views were superb. We also di not go to the other side of the the island to visit the famous beach call Giglio Campese. We were also told this was a great location. All are easily accessible from a public bus that runs hourly back and forth from Porto Giglio to Campese Beach stopping at the castle each way.
We wished we had taken an earlier ferry so we had more time to explore.
It is a great day trip and worth the visit if you are in the area.