I have the same problem of needing public transport around sydney.
I found a great searchable list of bush-walks around Australia at
http://wildwalks.com. if you use the advanced search, you can restrict it to specific types of public transport to the start and from the end, cirtuit vs one way, distance from a given city, etc.
for example, if you search for 2-4 hour circuit tracks with any public transport within 250km of Sydney, there are 83 walks you could use the Sunday weekend explorer and take 2 adults and 2 kids for $10.