A good question.
A local answer.

Use a market visit as the start of an unhurried neighbourhood walk. Browse the Adelaide Central Market when it is trading, then explore the surrounding streets for lunch rather than booking several separate attractions.

Choose the Adelaide Hills if you want a mix of scenery, village streets and food without making the entire day about cellar doors. Hahndorf is an easy focal point; leave room for one other stop instead of an ambitious circuit.

If your port call is short, Port Adelaide itself is an alternative to travelling all the way into the CBD. Its maritime history, heritage streets and waterfront make a coherent half-day; confirm the local connection and return timing before you leave the terminal.

Walk North Terrace and choose a couple of stops rather than rushing through every building. The Art Gallery of South Australia, State Library and Botanic Garden make a convenient cultural route; check whether a special exhibition needs a ticket.