Right in the middle of Sydney Harbour, Cockatoo Island features a bar, historic sites, and even allows you to camp during special periods like New Years Eve & Christmas.
The only way to get to cockatoo island is by Sydney Ferry which adds to the fun of the experience. Ferries to Cockatoo island leave from circular quay and darling harbour which makes getting there easy and a good activity that you can tack on to seeing other parts of Sydney. The ferry costs 5 or 6 bucks to get you to the island and about the same to get you back.
Cockatoo island is a unique experience; part cool bars, part historical wonderland there's something at cockatoo island for everyone.
The old shipping buildings on cockatoo island are amazing to look at and the abandoned warehouse feel of some hangers makes for great photography for those with a keen eye for photos.
There are tours available for those who want to be guided around cockatoo island but the island is easy enough to lead yourself around. Give yourself a couple of hours to explore the island if you're keen to check it out otherwise if you're just going for a beer in the sun, just leave a couple of hours of sunlight or two for you.