We have just returned from a cruise with MSC Grandiosa & visited La Goulette. You don't have to book a tour, there are plenty of taxi's waiting at the port. We payed 40 euro for a taxi for the 2 of us. We have been to Sidi Bou Said first & then to Carthage, this site is very big with many stops, amphitheater, bathing houses, villages where they lived,... To far to walk to all these sites, but the taxi driver waited for us. The tour was +- 4-5 hours & the taxi driver spoke a little English. We payed after we finished the tour.