Do not bother with a shore excursion just go left out of the port and walk a few hundred yards and enter the town by the gateway and you are quickly into the old town and its sights
We took the shuttle bus into the old town and it was well worth the visit. Lots to marvel at and wander through small alleyways. Will definitely go bak for longer visit.
There is a tram just to the left of the port which takes you to the main sites and was cheap and easy.
Not much here but shops and cages I would recommend staying on board if you have not booked an excursion.