Upper Colo Reserve is a lovely camping spot by the banks of the Colo River surrounded by the bushland and cliffs of Wollemi National Park. However rather than being part of the National Park it is actually operated by the Hawkesbury Council.
It is quite a large area with firepits, toilets and cold showers as well as a BBQ area and shelter. If you want to camp which I highly recommend the costs are $6 per adult and $3 per child over 5. The ranger comes around to collect the fees as it gets dark. But if you just want to visit during the day it doesn't cost anything. Also apparently the gates are locked at night but the cache isn't far from the road in any case.
Bookings are supposedly required for camping but I've never had a problem rocking up and finding a spare space. The full conditions are found at http://www.hawkesbury.nsw.gov.au/community/parks-and-recreation/upper-colo-camping?zoom_highlight=upper+colo
From here you can easily explore the area in a normal 2WD vehicle, though I would recommend 4WD if you want to do Mountain Lagoon Road up to Bilpin. There were severe washout sections where Low Range was a lot more comfortable crawling over them, but they didn't present any problems to the Suzuki.
Did you know that NSW has a geocaching association? Geocaching NSW aims to enhance and improve the activity of geocaching and holds regular events where geocachers meet to enjoy their common interests. Visit the association website here.