We did a ship's tour the first time in Tunis, but we aren't a great fan of these, so the last 2 times we have pre-arranged a local guide to meet us at the port. We told him what we wanted to do (or he could suggest). We recommend the Bardo Museum for the most exquisite mosaics, the souks (markets) in the old town and the famous artists village with the blue doors. You can probably fit 2 of these 3 things into the typical 4-5 hour port time.
We have used Batoua Voyages both times and were very happy. You can email them on firstname.lastname@example.org
. We paid EUR 134 for a guide, driver and air conditioned mini-bus, cheaper than the ships tours for 2 or more people.