There are certain touristy things that one must do while traveling. And some places are so great that even locals flock to them despite the fact that they're heavily trodden. The Opera Bar at the Sydney Opera House is one of those places.
There really isn't a good reason not to hit happy hour at the Opera Bar. The drinks are reasonably priced, and the selection is impeccable. There is live music every night of the week and the vistas offered by the bar are some of the best in all of Sydney.
Transportation options to and from the Opera House are plentiful not to mention that site-seeing and drinking almost always go good together.