The Ferry from Circular Quay to manly is beautiful boat ride. The Ferry leaves quite frequently around every 20min. When you are on the ferry you should sit outside and take in the scenery of the harbour.
Once you get to manly enjoy some time on the beach and for lunch grab some of Australia's best fish and chips at Fishmongers.
On your way back to the city stop in at Four Pines Brewery. Its a micro brewery very close to the Ferry stop. There you can try all of their in house made beers with their special tasting menu.
Once you have enjoyed your day in manly catch the Ferry back into the city.