Easy to buy a ferry ticket to Portofino just on the dock, where the tender comes in. You can buy a single or return. The ferry runs once every hour - the journey only takes about 15-20 mins. It is a picturesque trip. The coastline is beautiful and looking at all the expensive houses and villas is a sight in itself.
There's also a bus, but to be honest, the ferry is better as it takes you right into the centre of the port. I think you have to trek up a hill to get the bus back.
The Medina is a typical souk, labyrinthine streets, dealers offering to bargain, but not too aggressively. When we stepped down from the bus, a Tunisian woman greeted me happily with 'Bonjour', but not everyone was so friendly. Our guide said if we got lost, everyone would be able to tell us how to get to Kasbah. We had quite a bit of time to wander the souk on our own and we did end up lost. I asked various women (in French) for directions. Some just ignored me completely and others shrugged their shoulders, although they were obviously familiar with the area. Even a female police officer was vague and unhelpful. Another group of young women tried to help, however. The ship information said that if we didn't want to go to the ubiquitous carpet shop visit, we should say so, but I saw someone try this and the guide explained how difficult that would be for him, how he had to tell eveyone where to meet etc, etc. In the end, the couple just came to the demonstration. Having said that, it was more interesting than most, as we saw a woman at work, weaving a carpet and it was nice to see all the different designs.
Sidi Bou Said was a picturesque town, but I felt we spent too much time there. I would have liked to spend more time at Carthage or the Bardo museum. Nevertheless, a very informative and interesting tour.