When in Sydney, you may want to visit the Blue Mountains. If you have an OPAL card, you have all you need. Not only does OPAL cover the whole public transportation system from Wollongong in the south to Newcastle in the north, it also includes the local bus systems. So, for example, if you take the train to Katoomba, you also can use the town bus system.
The alternative is to buy an AU $40 ticket to the Blue Mountain Explorer bus, which goes to the same places and has pleasant drivers — but why spend the money when you already have transportation?