From the Crowne Plaza I'd walk down the road to Sky Tower - look at the views and orient yourself. It will also help you decide if weather is good enough to take the ferry across to Waiheke

If the weather is good I'd then walk down the hill to Waiheke ferry (opposite bottom of Queen St) and take that across. Head into the first small town on the island, wander around seeing the craft shops, wines etc. Lunch at either a cafe or vineyard. Then take ferry back - if it goes via Devonport (some ferries do, some don't) then hop out there and walk up Mt Victoria (small volcano behind the village). This will give great views across the Hauraki Gulf (including where you've just come from). Back in the city, dinner etc in Viaduct area.

If wet, then I second number_6 about the museum. From Sky City walk down Queen St and turn right at Customs St (1 back from waterfront) to take the link bus up to the museum. After seeing the museum, wander through the park surrounds (Domain) and explore Parnell. You could have dinner in Parnell, or back on the waterfront Viaduct area.

There is a market in Civic Square (not far from your hotel) which I think runs Fridays and Saturdays.