Hello OP,
Walk, walk, walk around the town. You can pay a little bit of money and walk on top of the city fortress wall. It's almost like an Escher painting.. where stairs leading down actually go up. It's a mountainous city. There's several restaurants you can stop and watch the world go by.
There's walking tours that you can take.. I believe I booked mine from the ship (Holland America, since you asked).
I think you'll have a wonderful day there.
Even though most of the people speak English, it's always nice to say something in Croat. I memorized "thank you" and you should have seen the smiles and grins from the Croats. "Fvalla."
Have fun.
David