If a half-day stop here is part of your cruise itinerary, it's probably easyest to do your own thing and not pay for a shore excursion. Transportation by cab is cheap and easy to get outside the docks area. You'll have time enough to go to the Walled City and wander around. But is that ALL you want to do in Croatia?

It is a very pretty city but since had only half a day,  the obvious touristy thing to do is walking along the city wall (12 EUR, overpriced, hot and sweaty).  You'll have time to cover about half of it and have lunch etc. The views of the harbour are quite spectacular.  You can have lunch under the ramparts, but you might find it expensive. So are the shops.