Although it has other functions (library, eateries etc) the best thing about the Customs House is the model of Sydney which is beneath a glass (?) floor so you can explore it in a few paces. Pick out where you are, where you've been and where you're staying as well as get a feel for the whole shape of the city. It's free to enter and provides an excellent indoor activity whether you wish to escape sun or rain. The Customs House is located just across the road from Circular Quay.